refine treatment of missing values

[FabianHofmann]

closes #144

From release notes:

• The _fill_value for LinearExpression and QuadraticExpression classes was changed to NaN for the constant array ("const"). This allows to use the where function for expressions with constant values in the argument other.
• The fillna function for variables was made more secure by raising a warning if the fill value is not of variable-like type.
• The where and fillna functions for expressions were made more flexible: When passing a scalar value or a DataArray, the values are added as constants to the expression, where there were missing values before. If another expression is passed, the values are added to the expression, where there were missing values before.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (cf62967) 87.59% compared to head (091933c) 87.70%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   87.59%   87.70%   +0.10%     
==========================================
  Files          15       15              
  Lines        3225     3244      +19     
  Branches      742      748       +6     
==========================================
+ Hits         2825     2845      +20     
+ Misses        290      288       -2     
- Partials      110      111       +1     

Files	Coverage Δ
linopy/variables.py	90.88% <100.00%> (+0.15%)	⬆️
linopy/common.py	91.25% <0.00%> (+0.41%)	⬆️
linopy/expressions.py	89.60% <88.88%> (+0.20%)	⬆️

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