Frontend: Deprecate OFP and purge from test base

[mlange05]

Note: Just testing for now...

As described in #209 , this PR deprecates the OFP frontend and adds a warning to the central OFP parse method. It also purges the OFP symbol from the outward-facing API and the test base (including the available_frontends() utiltiy), leaving internal mechanics to rely on the explicit mention of Frontend.OFP.

Please note that the frontend itself is still present and functional, but it's use is now discouraged.

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/411/index.html

[codecov]

Codecov Report

Attention: Patch coverage is 96.72131% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.04%. Comparing base (cc30c9e) to head (f6c2151).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
loki/frontend/ofp.py	66.66%	1 Missing ⚠️
loki/transformations/tests/test_pool_allocator.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #411      +/-   ##
==========================================
- Coverage   95.96%   93.04%   -2.92%     
==========================================
  Files         198      198              
  Lines       39320    39320              
==========================================
- Hits        37734    36587    -1147     
- Misses       1586     2733    +1147     

Flag	Coverage Δ
lint_rules	96.39% <ø> (ø)
loki	93.00% <96.72%> (-2.96%)	⬇️

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[mlange05]

Tests all passing, but coverage is decreased (as expected). Farewell old friend. 🫡

[reuterbal]

Thank you very much, this is fantastic and sad and soothening and so many mixed feelings...

As discussed offline, I have captured the indirect coverage regressions that are incurred from this in #406 (comment)

These should be taken care of when the factual removal of OFP is happening