* replaces mat / scalar with mat * scalar**-1.
* replace list argument to subs with a dict

(Although there are pending changes to SymEngine for the above two
issues, they aren't released in a version yet.)

* Removed deep as a kwarg to expand(). It doesn't do what we want
anyway.

* Disabled caching for tests in test_kernels. This exercises SymEngine
code generation every time the tests run. The execution time for these
tests without caching is about 6 minutes for me, so the overhead is
acceptable.

At least on my branch of SymEngine (v0.3.0), this test fails because
it greedily evaluates the Subs nodes.

Closes #16 on gitlab.

when using sympy, so code generation doesn't blow up.

Also add a test the uses LineTaylorLocalExpansion.

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.

This reverts merge request !31

This reverts merge request !26

When doing substitution, SymEngine seems to collapse Subs nodes. This
was leading to problems for sumpy, which relies on having Subs nodes in
the expressions after killing trivial assignments.

This change moves the job to pymbolic, which appears to preserve Subs
nodes.

This makes match_common_args() slightly better at detecting recursive
matches.

Closes #9

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