Optional argument to compute edge weights

[leonlan]

Closes #91.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (1750995) 100.00% compared to head (26706c2) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #98   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        16           
  Lines          256       257    +1     
=========================================
+ Hits           256       257    +1     

Impacted Files	Coverage Δ
vrplib/parse/parse_solomon.py	100.00% <100.00%> (ø)
vrplib/parse/parse_vrplib.py	100.00% <100.00%> (ø)
vrplib/read/read_instance.py	100.00% <100.00%> (ø)

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