Add in-memory cache for interpolation matrices

[sandorkertesz]

This PR adds an in-memory cache for the interpolation matrices. It can be controlled via the following methods:

• set_memory_cache()
• clear_memory_cache()
• memory_cache_info()

See: https://earthkit-regrid.readthedocs.io/en/develop/set_memory_cache.html#memory-cache

By default the cache size is 500 MB and uses the "largest" policy.

With this cache repeated interpolations between the same input and output grids can be run much faster. The speed-up is around x10 on a MacBook using the largest interpolation matrix available at the moment:

  earthkit.regrid.interpolate(data, 
                        {"grid": "O2560"}, 
                        {"grid": [0.1,0.1]})

See:

• https://earthkit-regrid.readthedocs.io/en/develop/examples/memory_cache.html

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.11594% with 19 lines in your changes missing coverage. Please review.

Project coverage is 92.59%. Comparing base (1b91b2f) to head (e4ec855).

Files with missing lines	Patch %	Lines
tests/test_memcache.py	94.11%	11 Missing and 5 partials ⚠️
tests/test_local.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #23      +/-   ##
===========================================
+ Coverage    92.03%   92.59%   +0.56%     
===========================================
  Files            7        8       +1     
  Lines          251      527     +276     
  Branches         7       15       +8     
===========================================
+ Hits           231      488     +257     
- Misses          18       32      +14     
- Partials         2        7       +5     

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[iainrussell]

This is a really great result @sandorkertesz!