Jitcxde-common

* Updates to documentation.
* Back-end for checks.

* Added `find_dependent_helpers`.

There now is a SymEngine version number to refer to as a dependency – which means that automatically installing this dependency should work and you do not have to build SymEngine on your own anymore.

* Switched from SymPy to SymEngine as a backend, which may speed up code generation by a factor of up to nine hundred. Almost everything that worked before should still work. See [SymPy vs. SymEngine](http://jitcde-common.readthedocs.io/en/latest/#sympy-vs-symengine) for details. As a required [Symengine release is pending](https://github.com/symengine/symengine.py/issues/204), you have to [build it from source](http://jitcde-common.readthedocs.io/en/latest/#building-from-source) for now.
* Added OpenMP support.
* Several improvements to documentation.
* Changed some default parameters and behaviours.
* Dropped support of some old, rarely used syntax.
* Dropped Python 2 support.

* `sympify` and `doit` are now applied to all input.
* Added a function for relative distance needed by JiTCDDE.

This will be probably the last release using SymPy for symbolics. The next one will use SymEngine.

This will also probably be the last release supporting Python 2.