Jitcode

* Improvements to documentation.
* Fixed wrong dependency in v1.3.0.

* Fixing some under-the-hood inefficiencies.

* 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.

* Slightly improved default choice of integrator parameters for Lyapunov exponents and Solve IVP.

* Added support for the integrators of [`solve_ivp`](http://scipy.github.io/devdocs/generated/scipy.integrate.solve_ivp.html).
* Some minor improvements to documentation.

* 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 handling in case of an unsuccessful integration to match JiTCDDE and JiTCSDE, i.e., an exception is raised.
* Changed some default parameters and behaviours.
* Dropped support of some old, rarely used syntax.
* Dropped Python 2 support.

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.