Closes #16 on gitlab.

This changes the P2P stage to be aware of self interactions in the FMM
and to exclude them, if desired. Self interactions are specified by
passing an extra array called *source_to_target* which matches source
numbers to target numbers.

These expansions use information from the PDE to compress the
number of needed Taylor coefficients.

- Add a H2DMultipoleExpansion class.
- Implement L2L, M2L, M2M operators.
- Teach loopy about casting to complex numbers (to handle a corner
  case where some multipole coefficients may look real valued).
- Update the tests.