Fix bug when saving Solution with terminal_currents

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[codecov]

Codecov Report

Merging #108 (c55e67a) into main (9661fb0) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
+ Coverage   93.97%   94.03%   +0.05%     
==========================================
  Files          41       41              
  Lines        4562     4574      +12     
==========================================
+ Hits         4287     4301      +14     
+ Misses        275      273       -2     

Files	Coverage Δ
superscreen/solution.py	94.33% <100.00%> (+0.50%)	⬆️
superscreen/solver/solve.py	86.38% <ø> (ø)
superscreen/test/test_transport.py	100.00% <100.00%> (ø)
superscreen/version.py	100.00% <100.00%> (ø)