Make the Scalar class compatible with symbolic phases

[RazinShaikh]

Please merge after #354

This PR modifies scalar.py to support symbolic phases. As such, it removes any explicit calculation of the complex scalar outside the dedicated to_number method.

The main change is due to the add_spider_pair method. The scalar added by this method is 1 + e^(i pi p1) + e^(i pi p2) - e^(i pi (p1+p2)) where p1 and p2 are phases of the two spiders. In the case when p1 and p2 are both non-clifford, there is no way of writing this scalar using scalar.phase and scalar.phase_node. So we introduce a new variable scalar.sum_of_phases which represents the term (c1*exp(i*phase1) + ... + cn*exp(i*phaseN)) as a dictionary mapping phase->coeffecient. Rest of the changes are in support of adding the sum_of_phases variable.

[jvdwetering]

These are some pretty complicated changes. Would it make sense to add a bunch of tests to make sure things work correctly? Also including all the various ways that the phase should be translated back into a string or saved to json?

[RazinShaikh]

Sorry, it's less complicated than it looks. The branch was split from the branch that fixed local complementation, etc so it had all the changes from that branch shown here again. Now I have merged the master branch onto this PR and the "files changed" went from 9 to 2.

[RazinShaikh]

But I am happy to add some tests for the changes in this PR

[jvdwetering]

Yes, this looks simpler now.
Btw, maybe not in this PR, but: is there some way in which we can instantiate a parameter in a diagram, including in the scalar? Like if I have some parameter alpha, and I then decide this is equal to pi/4, is there a way to make this happen on the graph and scalar?