add support for unprefixed doi.org urls

[AlexanderPico]

This would have cleared the last two sync actions we've seen

[codecov-commenter]

Codecov Report

Merging #21 (a4c7165) into main (8daf938) will not change coverage. The diff coverage is 0.00%.

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@@            Coverage Diff            @@
##               main      #21   +/-   ##
=========================================
  Coverage     63.03%   63.03%           
  Complexity     1102     1102           
=========================================
  Files            68       68           
  Lines          6999     6999           
  Branches        832      832           
=========================================
  Hits           4412     4412           
  Misses         2064     2064           
  Partials        523      523           

Impacted Files	Coverage Δ
...a/org/pathvisio/libgpml/model/GPML2013aReader.java	68.56% <0.00%> (ø)

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