fix: extend knmi importer with support for other projections

[mats-knmi]

A lot of metadata is currently hardcoded in the importer. To allow files in knmi hdf5 format that have different values here, we need to extend the importer.

[RubenImhoff]

Hi @mats-knmi, this is looking great! The tests still fail on the assertion of the right x- and y-grid coordinates. Is there still something wrong in the test grid (km instead of m), or is the assertion just too strict (with e-10)?

[codecov]

Codecov Report

❌ Patch coverage is 75.67568% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.13%. Comparing base (0d569d6) to head (7b14d56).

Files with missing lines	Patch %	Lines
pysteps/io/importers.py	75.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #562      +/-   ##
==========================================
- Coverage   84.18%   84.13%   -0.06%     
==========================================
  Files         170      170              
  Lines       14989    14992       +3     
==========================================
- Hits        12618    12613       -5     
- Misses       2371     2379       +8     

Flag	Coverage Δ
unit_tests	84.13% <75.67%> (-0.06%)	⬇️

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