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

Attention: Patch coverage is 72.74678% with 127 lines in your changes missing coverage. Please review.

Project coverage is 29.22%. Comparing base (93bbe4b) to head (694a5ba).
Report is 3 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   28.36%   29.22%   +0.86%     
==========================================
  Files          77       79       +2     
  Lines       18365    18991     +626     
==========================================
+ Hits         5209     5551     +342     
- Misses      13156    13440     +284     

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