LRUCache per field, not per file

[ChrisspyB]

A slightly more sensible cache. Quite a big change which affects plugin and server so:

• Test at scale with large/small cache on server, make sure it works
• Also check timings, the cache is now more fine grained so more time will be spent twiddling with it. Make sure this isn't now a bottleneck.
• Check memory footprint. In principle we should be able to make it quite large since jumpinfo objects tend not to be too big, though depends on packing (ccsds)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.18182% with 13 lines in your changes missing coverage. Please review.

Project coverage is 62.01%. Comparing base (ded3024) to head (0c24792).
Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/gribjump/info/InfoCache.cc	89.10%	11 Missing ⚠️
src/tools/gribjump-dump-info.cc	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #30      +/-   ##
===========================================
+ Coverage    61.77%   62.01%   +0.23%     
===========================================
  Files           85       85              
  Lines         4440     4454      +14     
  Branches       488      492       +4     
===========================================
+ Hits          2743     2762      +19     
+ Misses        1697     1692       -5     

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[ChrisspyB]

Consider a indexfile.load(offsets) option, which is like load but returns early if it has everything it needs.