coords_as_attributes functionality

[EddyCMWF]

This PR means that it is possible to convert coordinates/dimensions to variable attributes, only if the length of the coordinate = 1 (this requirement artificial and could be removed)

This means that it is possible to open variables which are on different levels (e.g. soil temperature variables) in a single dataset without loss of the metadata. i.e. without the change the depthBelowLandLayer is a coordinate of the dataset, therefore the different soil levels are incompatible. Now the depth is stored as t an attribute on each variable.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 41.39%. Comparing base (8b8e896) to head (5a9044c).

Files	Patch %	Lines
cfgrib/dataset.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #394      +/-   ##
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+ Coverage   41.26%   41.39%   +0.13%     
==========================================
  Files          26       26              
  Lines        2174     2184      +10     
  Branches      284      285       +1     
==========================================
+ Hits          897      904       +7     
- Misses       1277     1280       +3     

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[EddyCMWF]

Additional information, I chose to do it here as it will be useful to keep some level-types as dimensions (e.g. pressure/model level) but convert some level-types to attributes (e.g. soil-level/surface). Doing this earlier in the code meant that it would have been an all or nothing approach to including the vertical levels as a dimension