Add faster tests for graphical sequences.

Use linear time algorithm.
Add testing for directed, multigraphic, and pseudographic sequences.
Whitespace cleanup.
Change canonical name to is_graphical from is_valid_degree_sequence.
Small doc formatting fixes and updates.
Combine basic graphical setup into single function.
Change graphicality testing function names

Use is_digraphical(), is_pseudographical(), and is_multigraphical().
Add graphicality testing docs
Update documentation and formatting for Sphinx
Merge pull request #816 from hagberg/706-graphical

Linear-time graphical tests

Fixes #706