refactor(vtk): use pathlib, corrections to docstrings

[mwtoews]

This PR is primarily focused with the vtk export module:

• Ensure the vtk module works with pathlib and str path objects
• Use more pathlib throughout vtk module and test_export.py
• Many corrections to docstrings for vtk module
• Use Sphinx's deprecated directive with a link to the recommended method(s)
• Remove binary option with count_lines_in_file since it wasn't used and does not make sense
• Add is_binary_file and use it instead of "exists()" checks

[codecov]

Codecov Report

Merging #1603 (21d42f7) into develop (09e3bc5) will increase coverage by 0.0%.
The diff coverage is 76.1%.

@@           Coverage Diff           @@
##           develop   #1603   +/-   ##
=======================================
  Coverage     72.7%   72.7%           
=======================================
  Files          251     251           
  Lines        54488   54485    -3     
=======================================
- Hits         39650   39648    -2     
+ Misses       14838   14837    -1     

Impacted Files	Coverage Δ
flopy/export/vtk.py	69.8% <76.1%> (+<0.1%)	⬆️

[mwtoews]

Thanks @jdhughes-usgs I've used more with_suffix functions to simplify further.

Also, the docstrings are adjusted to use os.PathLike, which is a superclass of Path and other path-like objects (e.g. path), which could also be used (in theory).