DifferentialAlgebra: all doctests pass

All of the outstanding testsuite problems were due to syntax.

Three kinds of changes were made to pass the testsuite:

1. Expressions of the form "D[0](f)(x)" were converted to
   "diff(f(x), x)" using inverse_translate_str()
2. Failing tests were replaced with an equality test
   (i.e, "a == b" returns "True" instead of "a" returns "b")
3. Still failing tests were replaced with a string comparison
   (i.e, "str(a) == str(b)" instead of "a == b")