Performance Improvement

Update compute metrics to only calculate out degree fraction once and
then reuse results.
Performance Improvements #2

Moved to sparse matrix representation for out degree fraction (and
subsequent functions)
Performance Improvements

Performance improvements:
1) Convert to csc_matrix after matrix construction
2) Use builtin max/sum with axis=1 to push down computation into bumpy
3) Add note to flake out degree fraction which still needs to be
improved. Changed iteration so it (probably) no longer makes a copy of
each column (getcol definitely makes a copy).