GUI library isn't included in setup.py

[MichaelClerx]

This is one where we'll need to make some kind of trade-off.

The facts:

• Users installing with pip install manta-maastricht on a clean Python system will not get PyQt5, so manta won't work
• Users installing into an Anaconda environment or similar (again with pip) will probably have PyQt5 installed already, so for them it usually will work
• To make things consistent, we can add PyQt5 to the list of dependencies in setup.py. However... this only works on Python3. There's no pip install PyQt5 option on Python 2.
  • On the other hand, Python2 will only be supported until the end of the year
  • We could even think of doing something hacky like adding it to the dependencies only on Python3 systems...

Thoughts?

[MichaelClerx]

Have just updated the README with some instructions, so maybe we can just leave it like this for a few months, and then go the Python3-only route?