Feature/eckit stats

[pmaciel]

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Mirco, I've ported 90% of the mir statistics functionality here, it's not a full PR to be merged before much discussion but rather a draft.

For MIR, the requirements were:

• supporting finite and NaN missing values;
• Calculate statistics in time (for weather statistics and climatotlogical studies) and/or in space (supporting a variety of interpolation methods);
• Do online statistics, in partitioned objects that can be summed for global results (for parallel execution, although this was never taken advantage off);
• Support comparisons for pgen (which is a lot); And
• To support eccodes namespace "statistics" (grib_ls -n statistics), which is why there are spectral statistics too, but this isn't a requirement

I'm sure for other packages it will be different, let's see if they converge to something useful :-)

[pmaciel]

Again please do not merge this, before we have a wider discussion with other eckit devs.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 63.74%. Comparing base (9d2807d) to head (74e8b31).

Additional details and impacted files

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##           develop     #154   +/-   ##
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  Coverage    63.74%   63.74%           
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  Files         1066     1066           
  Lines        55360    55360           
  Branches      4086     4086           
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  Hits         35289    35289           
  Misses       20071    20071           

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