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.

Fix print statement in docs page

Use black formatting, update code style and conventions, miscellaneous pytest fixes