Make requirements for plotting optional

[hackermd]

At the moment the library cannot be used without installing graphical libraries, such as Matplotlib or Tkinter.

For many non-interactive use cases, plotting functionality is not required (for example in a HPC environment) and should therefore be optional in my opinion.

[keflavich]

Fair enough, I think that can be achieved. I'd welcome a PR to factor out the plotting utilities, otherwise I can try to get to it when I have time....

[davidedelvento]

Until that is done, the requirements.txt should be changed to have matplotlib included, rather than failing at runtime