Allow using to_target in xarry to GRIB conversion

[sandorkertesz]

This PR allows using to_target() to convert Xarray generated with the earthkit engine to GRIB. It also makes the to_grib() method on the earthkit accessor deprecated.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.55172% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.73%. Comparing base (67cc92d) to head (60f03f1).
⚠️ Report is 312 commits behind head on develop.

Files with missing lines	Patch %	Lines
tests/xr_engine/test_xr_write.py	95.34%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #730      +/-   ##
===========================================
+ Coverage    90.70%   90.73%   +0.02%     
===========================================
  Files          170      170              
  Lines        12949    13006      +57     
  Branches       618      625       +7     
===========================================
+ Hits         11746    11801      +55     
  Misses        1015     1015              
- Partials       188      190       +2     

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