added draw minima function to DisconnectivityGraph
modified disconnectivity graph centre function
modified disconnectivity graph centre function
Revert "modified disconnectivity graph centre function"

This reverts commit c406d7b6ce484acc2d85d1007b7ec8861c658cf2.
Revert "modified disconnectivity graph centre function"

This reverts commit a76b3f3d36077a9ceb8842ceb1bada8a4b5690f5.
reverted changes
add documentation to draw_minima()
Merge pull request #56 from smcantab/master

added draw minima function to DisconnectivityGraph