Use sympy's Indexed to represent pymbolics Subscript in interop mappers

See merge request !8

Better handling of the else branch

See merge request !11

I realized sympy.Piecewise iterates over the conditions
and returns the first which evaluates to True. So, the
else branch should have `True` instead of `sp.Not(cond)`
for a more robust implementation.

Implement pymbolics If statement through sympys Piecewise

See merge request !10

Have maxima parser recognize exponential functions

See merge request !9

Previous implementation of If through the EvaluationMapper
resulted in wrong behaviour. Although the Piecewise node
is no exact match for the if, it can be used to express it.

Maxima represents exp(x) as "%e^x". This enhances the
parser to recognize the special symbol %e and have any
power function with it as a base return exp instead of
pow.

They are no fit for symengine.

So far, pymbolic Subscripts could not be properly mapped to sympy.
There is a node Indexed in sympy though which does exactly what Subscript does.

Improve ConstantToNumpyConversionMapper: Allow to-numpy conversion of complex values with zero real part

Update to symengine-0.3.0

See merge request !7

Gitlab CI: Use maxima installed from Conda (closes #4 on gitlab).

Closes #4

See merge request !6

This reverts commit ef8097f5.

(https://github.com/conda-forge/ecl-feedstock/issues/5)

Fix few typos

Maxima osx fixes

Closes #3

See merge request !4