:bug: Rewriting an `IN.DAT` changes the results

[timothy-nunn]

Describe the bug

Looking at, #3892 I have noticed that reading in an IN.DAT and writing it out to NEW.IN.DAT changes the resulting MFiles.

Steps to reproduce

from pathlib import Path

from process.io.in_dat import InDat

DIR = Path(__file__).parent

st_regress = DIR / "st_regression.IN.DAT"
out = InDat(filename=str(st_regress))

out.write_in_dat(
    output_filename=str(DIR / "new.IN.DAT")
)  

Next, run both files and compare the results (I tested this using the st_regression.IN.DAT in the regression test suite and got 362 differences)

Expected behaviour

The IN.DATs are the same, as are the resulting MFiles.

[timothy-nunn]

Expected fix:

1. A test to check that reading and re-writing an IN.DAT does not change the IN.DAT.
2. Fix the source of the difference between the MFiles