Checkpoint differentials.

Performance issues in integral_basis need to be addressed
before further testing of differentials.
Replace simplify() with powsimp() in puiseux.py:base_expand

For performance purposes, we perform a powsimp() instead of
a full simplify. This is because we know that the final
expression should be a Puiseux series in T. Note that this
method does not simplify the coefficients of T all that
well. However, it does result in on average better
performance when running integral_basis().
Improve performance of integral_basis().

We implement a more refined use of Sympy's expand()-based
functions by taking advantage of the fact that our
expressions only include rational powers of symbols as
well as symbolic imaginary components. This resulted in
an approximate 30% speed up.