Allow better injection of data holding container choices

[znicholls]

It turns out that your data holding container can make a huge difference for ease and speed of e.g. model running and cost calculations. At the moment, our stack is very scmdata focussed, which isn't always the right choice. We should go through the stack and clear out this tight coupling to scmdata as much as possible.

[znicholls]

While doing this:

• move optional dependencies to actually being optional (e.g. scmdata, plotting dependencies)
• install all optional dependencies for the tests done in Move to copier template #27
  • rationale: we want the tests to run with maximum coverage
    • also push this to copier