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.