add per_unit keyword with io

[jjstickel]

This small change enables the network to be read in without setting the network to per_unit = true. It is useful for creating visualizations.

[codecov]

Codecov Report

Merging #142 (3f29c4e) into master (ac81eda) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #142   +/-   ##
=======================================
  Coverage   91.37%   91.37%           
=======================================
  Files          37       37           
  Lines        4058     4059    +1     
=======================================
+ Hits         3708     3709    +1     
  Misses        350      350           

Impacted Files	Coverage Δ
src/io/common.jl	96.66% <100.00%> (+0.11%)	⬆️

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[tasseff]

This should be a good compromise for the moment. Thank you!