Rename nodesimg to nodes, allow for nodes_df and edges_df input

[wiheto]

Improvement to make input more unified.

• [x] nodesimg should be specified using nodes. If nodes is a dictionary, valid path, or nifti, assume it is nodesimg.
• [x] nodes_df should be allowed to supplement a nodsimg file for colour/size/highlighting. Implementation: Depends how the above is implemented
• [x] edges_df should also be allowed for when the input is a numpy array. Implementation: if edges is numpy array, edges_df outer merges with it after it is converted edges.merge(edges_df, how='outer').

[wiheto]

All these features are now included in develop_0.2.0 PR #40