Move visualisation to an external module

[Stefan Pfenninger]

Problem description

To reduce the complexity of the Calliope core code, we want to move visualisation to an external module.

This external module could be based on using friendly_data and could move to greater use of matplotlib rather than plotly.

TODO:

• Remove plotting from core (#367)
• Add plotting functionality back in @ https://github.com/calliope-project/cview
• Change how we describe plotting in the documentation to point to this new module

[Stefan Pfenninger]

What's remaining for now is to update the main calliope documentation.