This change makes `Op.perform` a mandatory method.

Since more than a few `Op`s do not have Python implementations, they've been
made to extend `_NoPython*Op` classes that provide an `Op.perform` that simply
raises a `NotImplementedError`.

This causes `abstractmethod`-decorated methods to take effect (i.e. classes that
don't implement these required methods will fail during construction).

This commit also includes numerous indirect-to-direct `theano.gof` import
corrections.

This commit removes most `try` logic that relies on `MethodNotDefined`
exceptions to determine which `Op`s and `Type`s actually have C
implementations.

Some complementary changes that were needed along the way:
- Updates to the CLinkerType interface to include an optional `c_compiler`
argument.
- Provide no-op default implementations for `CLinkerObject`, `CLinkerOp`, and
`CLinkerType` methods.

Set HTML writer version in conf.py to workaround sphinx whitespace issue.

* Implement improved Softplus

* Extend gradient test to wider range

* Add numerical accuracy test

* Fix indentation

This change prevents Python build changes (e.g. CPython -> PyPY) during
dependency installations.

They just wrapped around numpy.seterr, which users should call directly instead.
This also changes the default behaviour from setting numpy.seterr(all="ignore") to leaving all at its numpy default.

* Add how to add JAX Ops rst doc

* Update doc/JaxOps.rst
Co-authored-by: Thomas Wiecki <thomas.wiecki@gmail.com>
Co-authored-by: Thomas Wiecki <thomas.wiecki@gmail.com>

Closes #55

Closes #203

And specify their purpose and public API.

Closes #23.