Jitcsde

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.
* Fixed a nasty memory leak.
* Changed some default parameters and behaviours.
* Dropped support of some old, rarely used syntax.
* Dropped Python 2 support.

* Fixed a crucial bug in the integrator (for non-additive SDEs) that passed unnoticed due to a bug in a test.

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.

* Some fixes to documentation.