Epydemic

- Fixed issue with SynchronousDynamics
- Added cookbook recipe on keeping access to the end-state network

- Added network statistics collection as a process
- Refactored Process and Dynamics so that the dynamics maintains the
working copy of the network rather than the process
- Added process sequence combinator
- Added tutorial on sequencing vs sub-classing processes
- Fixed bug in Monitor that only appeared when sequencing

- Refactored Percolate process to allow specialisation of behaviour for occupied
and unoccupied edges

- Refactored Dynamics to introduce NetworkExperiment, where there's no process per se
- Added powerlaw-with-cutoff as a standard network generator
- Added Newman-Ziff algorithms for exploring percolation
- Very basic Newman-Ziff tests, need significant work
- Added script to create percolation diagrams

- Added percolation process
- Backported to support versions of Python from 3.6, and hence also PyPy3

- Removed support for Python 3.6 and 3.7 (since we need the annotations
future and types features only supported from Python 3.8)