Digraph Arrows to fix #2757 (#2760)

* changing source node markers, not working

* markers set by the angles between two nodes, not working

* Using FancyArrowPatch seems to work

* Patch with if and elif cases, working

* Docstring and magic numbers fix

* Add final comments

* Final 1st draft

* Arrow to maker's edge: circle marker only

* Add arrow size, and possibility of many marker sizes

* add arrow color for array of numbers

* return edge_collection and None when arrows

* return arrow_collection list

* add arrow example in doctring

* fix comment lengths

* fix doctring draw_networkx_edges

* fix docstring

* Fix spring_layout typo in docstring and old pep8 line length errors