Delete demo on Solid body rotation with multiple prognostic variables

[ddundo]

I think that the solid_body_rotation_split.py demo is written in a non-goalie way, which became apparent when working on #169. For example, the way it's written would not allow us to extend it to a goal-oriented approach.

It's also a very very specific case where all PDEs are the same. So I'm not sure how useful it is in general, and may even be misleading/confusing. E.g. I think that adding the field kwarg to get_form may be quite confusing to new users.

I would either:

• rewrite it in a goalie way - i.e. without the shortcut for different fields
• or remove it completely, since we have gray_scott_split.py to demonstrate how to deal with multiple variables (which I think is the point of the solid_body_rotation_split.py demo

What do you think @jwallwork23?

[jwallwork23]

Sure, feel free to delete it.