support x parity constraints #72
Description
Right now we only support y and z parity. The reason for this is that, on distributed-memory machines, the x dimension is distributed and doing the parity constraint is painful because it requires communication.
However, it would be straightforward to implement an x-parity constraint that worked only in serial mode and gave an error for parallel calculations.
The relevant code for y and z parity is in:
https://github.com/stevengj/mpb/blob/master/src/maxwell/maxwell_constraints.c