work with cBioPortal on cancer mutation-gene link set for CyTargetLinker

[egonw]

discussing this with @inodb here at #GSoCSummit2019

[mkutmon]

cool idea. just saw they have a web API. I fear the dataset would be too large for a CyTargetLinker-like link set (might be wrong about that!), but we could try to implement an additional handler in CyTargetLinker to get the data from cBioPartal directly when extending a network.