METK-171: Add API for mars key parsing

[caraghbiner]

Description

There are a number of environments (e.g. in the CDS, in polytope, ...) where python scripts need the expansion functionality of metkit (e.g. 1/to/10/by/1 --> [1,2,3,...]). This functionality is currently only exposed via a MarsRequest object in pymetkit. However, in these environments requests have keys that are not part of the mars language (e.g. "feature" in polytope), but yet still need to do a mars-like expansion on a subset of the keys.

As a result, everyone just implements their own python version of how they think the language expansion behaves for the subset of keys they are interested in. This results in lots of code duplication and wasted effort. It is also likely to be incorrect for the more complex keys: it is not really possible to do a python implementation from scratch of context dependent expansions (e.g. param) without using the metkit language yaml files. These files also update frequently, so other implementations quickly become stale.

I propose we add some plumbing in (py)metkit to support the expansion of mars keys without requiring the construction of a mars request object.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.40%. Comparing base (97063d8) to head (e6341f9).

Files with missing lines	Patch %	Lines
src/metkit/mars/MarsLanguage.cc	0.00%	17 Missing ⚠️
src/metkit/api/metkit_c.cc	0.00%	14 Missing ⚠️
src/metkit/mars/MarsExpansion.cc	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #236      +/-   ##
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- Coverage    50.51%   50.40%   -0.11%     
===========================================
  Files          385      385              
  Lines        15438    15471      +33     
  Branches      1307     1312       +5     
===========================================
  Hits          7798     7798              
- Misses        7640     7673      +33     

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