'Cleanup' filters for the graph

[narnolddd]

Would be awesome to have some cleanup functions on the graph/graph view, my suggested main ones would be:

• self-loop filter: remove all self-loops.
• largest connected component: retain just the largest component of g
• remove isolates (remove nodes not connected to any other node)

Would firstly just be a nice thing to have for users manipulating the graph. Another reason this might be handy is that some algorithms require something of the graph (e.g. for it to have no self-loops or to be connected etc) and currently you have to handle that logic on a case-by-case basis e.g. in the motifs algorithm you need to have self-loop checking in all the subroutines.