IO: multiindeces and general fixes

[FabianHofmann]

• The IO with NetCDF files was made more secure and fixed for some cases. In particular, variables and constraints with a dash in the name are now supported (as used by PyPSA). The object sense and value are now properly stored and retrieved from the netcdf file.
• The IO with NetCDF file now supports multiindexed coordinates.
• The representation of single indexed expressions and constraints with non-empty dimensions/coordinates was fixed, e.g. x.loc[["a"]] > 0 where x has only one dimension. Therefore the representation now shows the coordinates.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (b437f93) 89.17% compared to head (2ac42c5) 89.23%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #202      +/-   ##
==========================================
+ Coverage   89.17%   89.23%   +0.05%     
==========================================
  Files          15       15              
  Lines        3298     3343      +45     
  Branches      757      768      +11     
==========================================
+ Hits         2941     2983      +42     
  Misses        248      248              
- Partials      109      112       +3     

Files	Coverage Δ
linopy/common.py	92.50% <ø> (+0.83%)	⬆️
linopy/constraints.py	90.66% <100.00%> (-0.51%)	⬇️
linopy/expressions.py	89.44% <100.00%> (-0.31%)	⬇️
linopy/testing.py	100.00% <100.00%> (ø)
linopy/io.py	88.32% <95.12%> (+0.41%)	⬆️

... and 2 files with indirect coverage changes

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