Fixes/improvements to get_dot_dependency_graph().

See merge request !14

- Fix a typo in an argument name.
- Reverse the dependency edges, because it's more typical for the
  arrow to point to the instruction that the computation depends on,
  rather than the other way around.
- Add a hook for customizing node attributes.

Fix BitwiseAnd stringifier (&, not ^)

See merge request !13

Support sympy.UnevaluatedExpr

See merge request !12

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