Co-authored-by: Brandon T. Willard <brandonwillard@users.noreply.github.com>

Quick fix for import issues introduced by #21

Introduce a JAX Linker class

Doing this enables general use of `jax` >= 0.2.0; however, we still need to find
a better fix for the shape-related problems introduced by omnistaging.

Closes #43.

Closes #10.

Well, at least the JAX part, but Cython and Numba implementations can follow
very similar approaches and accomplish the same thing.

This is a cheap work-around for binary operators that actually behave as
variadic operators.  See https://github.com/pymc-devs/Theano-PyMC/issues/26.

* Allow single dimension slices in mgrid and ogrid

* Use requirements.txt and pin black version

* Update black formatting

* use sphinx section names

* Generalize the type interface to tensor.zeros and tensor.ones

* Add axis argument to squeeze

* remove optimization from ScalarSigmoid.c_code()

Remove the optimization from ScalarSigmoid.c_code(), as it was incorrect and did not optimize much. Add test case that passes without that optimization and failed with it.
Co-authored-by: Brandon T. Willard <brandonwillard@users.noreply.github.com>
Co-authored-by: Oriol (ZBook) <oriol.abril.pla@gmail.com>

Update requirements and pin black version

Refactor Travis config and add Coveralls

This can be removed after CI is in working order.

Accompanying this commit are a few test fixes (e.g. enable previously disabled
tests, remove tests with incorrect assumptions, skip tests earlier, mark more
tests as slow, etc.) and some minor refactoring.