Replace numpy isscalar check by pandas isscalar check

[FabianHofmann]

closes #59

[codecov]

Codecov Report

Base: 86.76% // Head: 86.76% // No change to project coverage 👍

Coverage data is based on head (3c34204) compared to base (131edf1).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #65   +/-   ##
=======================================
  Coverage   86.76%   86.76%           
=======================================
  Files          10       10           
  Lines        2055     2055           
  Branches      327      327           
=======================================
  Hits         1783     1783           
  Misses        204      204           
  Partials       68       68           

Impacted Files	Coverage Δ
linopy/variables.py	91.63% <100.00%> (ø)

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