4174 only read indats using the indat class

[clmould]

Description

• Moves validate_input to the InDat class and rename to check_obsolete_variables
• parse_input_file no longer re-reads the file line by line and instead uses the data read in by InDat
• Fix PLR1702 errors in these files

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 79.05405% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.04%. Comparing base (620d1e9) to head (4149387).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
process/core/io/in_dat/base.py	59.72%	29 Missing ⚠️
process/core/input.py	97.05%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4523      +/-   ##
==========================================
+ Coverage   49.88%   50.04%   +0.16%     
==========================================
  Files         151      151              
  Lines       29871    29935      +64     
==========================================
+ Hits        14901    14981      +80     
+ Misses      14970    14954      -16     

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