Change of docs for shunts in network data format

[LKuhrmann]

Changed the network data format shunt description from

"gs":<float>,      # Active power withdrawn per voltage p.u.
"bs":<float>,      # Reactive power withdrawn per voltage p.u.

to

"gs":<float>,      # Active power withdrawn at unity voltage (1 p.u.)
"bs":<float>,      # Reactive power injected at unity voltage (1 p.u.)

Reduces the confusion from #978.

Also changed from per voltage p.u. to at unity voltage (1 p.u.) as the current phrasing implies a false linearity with voltage (it is quadratic).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.88%. Comparing base (c404096) to head (a885568).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #979   +/-   ##
=======================================
  Coverage   93.88%   93.88%           
=======================================
  Files          43       43           
  Lines        9654     9654           
=======================================
  Hits         9064     9064           
  Misses        590      590           

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