Rewrote the changes to lambdify

Now in lambdastr, the default behavior is not to dummify everything; it is a
kwarg option. When lambdstr is called by lambdify though, dummify is set to
True. The justification for this is that if lambdastr is called directly by a
user, they might want to observe the output, in which case all Dummy symbols
would be confusing; if lambdify calls lambdastr, they won't be able to see the
output, and the lambdastr readability doesn't matter, making the increased
range of input arguments worth it.

In addition, translations from ImmutableMatrix -> matrix needed to be added, as
calling sympify on a Matrix results in an ImmutableMatrix.

A few tests were added to ensure that the new lambdify functionality works.