Handle degenerate bifurcation case. Many thanks to Pieter Kitslaar for
the hint, https://github.com/pkitslaar/vmtk/commit/167200c063c65813a7c4fc07f232474a8ff683ad
Remove spurious numpy import
Merge pull request #93 from lantiga/master

Fixed handling degenerate bifurcation case in bifurcation reference systems. Thanks to Pieter Kitslaar for the hint.