Convert process initialisation routines to python

[timothy-nunn]

PROCESS initialisation sequence Fortran -> Python

Iteration variable initialisation sequence Fortran -> Python

initial.f90 post-input pre-model call validation Fortran -> Python

The input validations raise a ProcessValidationError, a new exception introduced specifically for input validation errors in a first move away from the in-built error handling system. As a temporary measure, the non-level 3 errors use warnings.warn until a permanent warning solution is found.

This does mean that our errors and warnings will temporarily operate through two systems, however this is no different how it was in Fortran, with inconsistent use if print/write statements and stop 1 statements.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.74678% with 127 lines in your changes missing coverage. Please review.
✅ Project coverage is 30.65%. Comparing base (36523e6) to head (65e198e).
⚠️ Report is 564 commits behind head on main.

Files with missing lines	Patch %	Lines
process/init.py	73.82%	117 Missing ⚠️
process/exceptions.py	50.00%	7 Missing ⚠️
process/main.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3406      +/-   ##
==========================================
+ Coverage   29.63%   30.65%   +1.02%     
==========================================
  Files          78       80       +2     
  Lines       19105    19567     +462     
==========================================
+ Hits         5661     5999     +338     
- Misses      13444    13568     +124     

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[jonmaddock]

Great, this is a nice solution.