Add a to_xarray converter

[coroa]

Please confirm that this PR has done the following:

• [ ] Tests Added
• [ ] Documentation Added
• [ ] Name of contributors Added to AUTHORS.rst
• [ ] Description in RELEASE_NOTES.md Added

Adding to RELEASE_NOTES.md (remove section after adding to RELEASE_NOTES.md)

Please add a single line in the release notes similar to the following:

- (#XX)[http://link-to-pr.com] Added feature which does something

Description of PR

Add a simple utility method to convert a pyam dataframe into a xarray DataArray with a configurable set of stacked dimensions (so as not to add too many nan's).

This is a draft to get feedback on the judged utility of having something like this.

Possible points for improvement:

• [ ] Units could use pint-xarray
• [ ] Meta data handling would be nice.
• [ ] IamDataFrame.from_xarray classmethod or IamDataFrame.init code path for DataArray

[codecov[bot]]

Codecov Report

Merging #619 (b0e3c4c) into main (21ada20) will decrease coverage by 0.2%. The diff coverage is 27.2%.

@@           Coverage Diff           @@
##            main    #619     +/-   ##
=======================================
- Coverage   94.3%   94.1%   -0.3%     
=======================================
  Files         54      54             
  Lines       5444    5462     +18     
=======================================
+ Hits        5138    5142      +4     
- Misses       306     320     +14     

Impacted Files	Coverage Δ
pyam/utils.py	88.6% <23.8%> (-3.3%)	:arrow_down:
pyam/core.py	94.6% <100.0%> (+<0.1%)	:arrow_up:

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[gidden]

Closing this for now for lack of work, @coroa can pick it up again if he'd like to