feat(datafile): add .headers property with data frame

[mwtoews]

This feature adds a .headers property for a pandas data frame of the headers of various data files, including HeadFile, FormattedHeadFile, UcnFile and CellBudgetFile. This is a modern accessor to the headers, which is created internally using:

pd.DataFrame(self.recordarray, index=self.iposarray)

where the index is the file position to the start of each array. Text fields are decoded to str types (rather than dealing with bytes types, which is probably a hang-over from Python2). Int32 types are left as-is, except for iposarray which should be int64 to read large files >2GB. Float32 types are also left as-is, since these would otherwise have a lossy conversion to float64.

With the CellBudgetFile, the headers vary depending on the type of budget file. For instance, "classic" files (created without "COMPACT BUDGET" option) don't have imeth, delt, pertim, or totim columns. Furthermore, only files with imeth=6 have the extra text columns modelnam, paknam, modelnam2, and paknam2, since these fields are always empty.

This PR also adds more checks to the outputs created with ._build_index() methods. Eventually, I'm planning to deprecate other properties and functions that are using the .recordarray structured array, so it is important to have a detailed trace of these outputs before replacing them.

This PR also moves the __enter__ / __exit__ methods from BinaryLayerFile to LayerFile (this feature was from #669). This is so FormattedHeadFile can also use the "with" context statement to also auto-close the file.

---

Examples

E.g. show data frames to markdown with:

python -c "from flopy.utils import HeadFile; print(HeadFile('examples/data/mf6/create_tests/test004_bcfss/expected_output/bcf2ss.hds').headers.to_markdown())

examples/data/mf6/create_tests/test004_bcfss/expected_output/bcf2ss.hds

	kstp	kper	pertim	totim	text	ncol	nrow	ilay
52	1	1	1	1	HEAD	15	10	1
1304	1	1	1	1	HEAD	15	10	2
2556	1	2	1	2	HEAD	15	10	1
3808	1	2	1	2	HEAD	15	10	2

examples/data/mt3d_test/mf2kmt3d/reinject/MT3D001.UCN

	ntrans	kstp	kper	totim	text	ncol	nrow	ilay
44	2	1	1	25	CONCENTRATION	46	31	1
5792	4	1	1	50	CONCENTRATION	46	31	1
11540	6	1	1	75	CONCENTRATION	46	31	1
17288	8	1	1	100	CONCENTRATION	46	31	1
23036	10	1	1	125	CONCENTRATION	46	31	1
28784	12	1	1	150	CONCENTRATION	46	31	1
34532	14	1	1	175	CONCENTRATION	46	31	1
40280	16	1	1	200	CONCENTRATION	46	31	1
46028	18	1	1	225	CONCENTRATION	46	31	1
51776	20	1	1	250	CONCENTRATION	46	31	1
57524	22	1	1	275	CONCENTRATION	46	31	1
63272	24	1	1	300	CONCENTRATION	46	31	1
69020	26	1	1	325	CONCENTRATION	46	31	1
74768	28	1	1	350	CONCENTRATION	46	31	1
80516	30	1	1	365	CONCENTRATION	46	31	1

examples/data/mfusg_test/01A_nestedgrid_nognc/output/flow.cbc

	kstp	kper	text	ncol	nrow	nlay
36	1	1	CONSTANT HEAD	121	1	1
556	1	1	FLOW JA FACE	601	1	1

examples/data/mfusg_test/03A_conduit_unconfined/output/ex3A.cbb

	kstp	kper	text	ncol	nrow	nlay	imeth	delt	pertim	totim
52	160	1	STORAGE	100	100	-2	1	1	160	160
80104	160	1	CONSTANT HEAD	100	100	-2	2	1	160	160
80160	160	1	FLOW RIGHT FACE	100	100	-2	1	1	160	160
160212	160	1	FLOW FRONT FACE	100	100	-2	1	1	160	160
240264	160	1	FLOW LOWER FACE	100	100	-2	1	1	160	160

examples/data/mf6/create_tests/test004_bcfss/expected_output/bcf2ss.cbb

	kstp	kper	text	ncol	nrow	nlay	imeth	delt	pertim	totim	modelnam	paknam	modelnam2	paknam2
64	1	1	FLOW-JA-FACE	1700	1	-1	1	1	1	1
13792	1	1	WEL	15	10	-2	6	1	1	1	GWF_1	GWF_1	GWF_1	WEL-1
13976	1	1	RIV	15	10	-2	6	1	1	1	GWF_1	GWF_1	GWF_1	RIV-1
14272	1	1	RCH	15	10	-2	6	1	1	1	GWF_1	GWF_1	GWF_1	RCH-1
19176	1	2	FLOW-JA-FACE	1700	1	-1	1	1	1	2
32904	1	2	WEL	15	10	-2	6	1	1	2	GWF_1	GWF_1	GWF_1	WEL-1
33168	1	2	RIV	15	10	-2	6	1	1	2	GWF_1	GWF_1	GWF_1	RIV-1
33464	1	2	RCH	15	10	-2	6	1	1	2	GWF_1	GWF_1	GWF_1	RCH-1

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.4%. Comparing base (d81d7c0) to head (8dc1bc4).
Report is 6 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2221     +/-   ##
=========================================
+ Coverage     69.8%   70.4%   +0.5%     
=========================================
  Files          294     294             
  Lines        59008   59025     +17     
=========================================
+ Hits         41228   41578    +350     
+ Misses       17780   17447    -333     

Files	Coverage Δ
flopy/utils/binaryfile.py	81.8% <100.0%> (+0.3%)	⬆️
flopy/utils/datafile.py	72.6% <100.0%> (+0.5%)	⬆️
flopy/utils/formattedfile.py	88.9% <100.0%> (+0.2%)	⬆️

... and 36 files with indirect coverage changes