Trac: Tidy up Python implementation

[MichaelClerx]

https://chaste.cs.ox.ac.uk/trac/ticket/2458

Various tasks left over from https://chaste.cs.ox.ac.uk/trac/ticket/2390, that aren't significant enough to warrant their own tickets:

• Enforce use of an output folder when generating temporary files; perhaps make it a constructor argument.
• Move the compact syntax parser into the fc package (the script called by C++ can set an environment variable so the module knows what to import).
• Improve docstrings (mostly copying from the C++ versions). See also Add module level docstrings #16.
• Also document clearly how to install the required third-party libraries
• Make Compile indicate whether to evaluate the string using numexpr or eval
• Check (& maybe fix) behaviour of re-running python tests when imported modules change
  • This is broken for the Python FC implementation at least. It might be that SCons' scanner doesn't understand the relative within-package imports.