Delete broken/obsolete h2dterms and l2dterms wrappers, and replace

Closes #39

See merge request inducer/sumpy!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 inducer/sumpy!114

Rewrite complex coefficients that cannot be represented using single precision

See merge request !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 inducer/sumpy!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 !111

Conda: Use Python 3.6

See merge request !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 inducer/sumpy!106

Help CSE in l2p

See merge request !104