(Closes #3435) Metatrans docs

[LonelyCat124]

This is the initial stage to have support for docs from _SUB_TRANSFORMATIONS supported.

There are remaining issues to fix:

1. No unit tests cover this code.
2. We can get repeated options which result in repeated docs:

>>> print(MaximalOMPParallelRegionTrans.apply.__doc__)
    Applies the transformation to the nodes provided.


    :param nodes: can be a single node, a schedule or a list of nodes.
    :param bool node_type_check: whether or not the type of the
        nodes enclosed in the region should be tested to avoid using
        unsupported nodes inside a region.
    :param bool node_type_check: (Option used for OMPParallelTrans) whether or not the type of the
        nodes enclosed in the region should be tested to avoid using
        unsupported nodes inside a region.

MaximalOMPParallelRegionTrans inherits the node_type_check from RegionTrans, but then also finds it imported from its SUB_TRANSFORMATION class. @sergisiso Do you think I should avoid repeated docstrings? Should it be limited in some way to only be if the strings are ==? I think I can do that check (as the "Option used for ...") is added later.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (09431a7) to head (275003d).

Additional details and impacted files

@@            Coverage Diff             @@
##            master     #3438    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          392       392            
  Lines        54791     54903   +112     
==========================================
+ Hits         54791     54903   +112     

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[LonelyCat124]

@sergisiso This should be ready for a first look now. Also let me know if you're happy with all the behaviour shown in test_transformation_doc_wrapper_subtrans from utils_test.py

[sergisiso]

I am happy with the behaviour @LonelyCat124, but the implementation is a bit hard to follow, specially the utils.py could benefit with more clear typehints and docstrings.

[LonelyCat124]

@sergisiso Ready for another look, most comments addressed, a couple I've just responded to with comments/questions for this review cycle.

[sergisiso]

@LonelyCat124 This is approved for merging, I end up doing the final suggestions myself since only involved comments and formatting, but let me know if you disagree with anything.