Added a nonzero diagonal to classical strength measure, had to
change direct interpolation and splitting as well to handle
strength matrices with a nonzero diagonal.
Added a nonzero diagonal to the symmetric strength measure and changed
the unit tests
Added a unit test for remove_diagonal, and added text about the
desired standard strength interface
Unified the strength of connection returns for classical,
symmetric, and distance
Unified the strength of connection return values for evolution measure,
and algebraic distances.  Also, added support for all the strength
measrues in classical.
One more change to make evolution measure work with classical.py
Updated todo list to reflect work done on strength of connection