Jitcdde

Minor stuff:

* minor performance improvements,
* minor bugfixes,
* documentation improvements,
* better error messages and warnings.

* Fixed a bug in the error estimation. This bug lead to errors being overestimated for time steps smaller than 1 and underestimated for time steps larger than 1. As a result, dynamics with time scales smaller than one were integrated overly accurately (and slowly), while dynamics with larger time scales were integrated less accurately than intended. It is unlikely, though not impossible, that this had relevant impacts on results where the step size was larger than 1. There is a reason why I did not notice this bug earlier.

* `step_on_discontinuities` now adapts time steps.

* `generate_lambda` now allows to control simplification.

* Some improvements to documentation.

* Implemented callbacks.
* Some performance improvements by using write-only arrays (which may require to make a copy of some JiTCDDE-generated array before writing to it).

* Added support for input.
* Added support for neutral DDEs, i.e., allowed differential equations to depend on the past derivative. This is still somewhat experimental, but seems to work.
* Control parameters can now be passed as an iterable.
* Added SymPy symbols for `y` and `t` etc. (as an alternative to SymEngine symbols).
* Implemented quick installation test.
* Several improvements to error messages, warnings, and documentation.

* Added capability to insert jumps of the state.
* Vastly reorganised how the past is represented under the hood. You can now dissect and modify the past using [the new CHSPy module](https://chspy.readthedocs.io).

* Fixing issue 17 due to updates of SymEngine.
* Added quick installation check.
* Improved error messages.