Fix biharmonic kernel expressions

See merge request !117

Delete broken/obsolete h2dterms and l2dterms wrappers, and replace

Closes #39

See merge request !115

with FMMLib-based expansion order finder

Closes #39

- [ ] Point requirements.txt back to pyfmmlib master after pyfmmlib!15 is merged

tools: fix deprecation

See merge request !114

Rewrite complex coefficients that cannot be represented using single precision

See merge request inducer/sumpy!113

Loopy stops with an exception in this case, asking the user to explicitly cast to
np.complex64/128

Fix deprecated temp_var_type usage

See merge request !107

Add a direct test of expansion toy infrastructure

See merge request !112

Tests the most important functionality (P2E, E2P, E2P).

Adapted from the 3D paper experiments.

CI: Stop targeting specific Py3 versions

See merge request inducer/sumpy!111

Conda: Use Python 3.6

See merge request inducer/sumpy!109

This fixed recent Conda failures due to lack of a version of pyopencl
2018.2.2 by moving from Python 3.5 to Python 3.7.

This reverts commit da0a6c31.

This reverts commit 2a8a250f.

This reverts commit 4bc2a79e.

This reverts commit 11e0f7fc.

CI: Drop Py35, add Py37

See merge request !106

Help CSE in l2p

See merge request !104