Add type and field checks to `MeshSeq`

[ddundo]

Closes #169.

This PR:

• adds MeshSeq._outputs_consistent and calls it in __init__. The method checks that function spaces, initial conditions, and forms are given in a dictionary format with MeshSeq.fields as keys
• adds unit tests for above
• tidies up some other tests

The only problem is that now solid_body_rotation_split.py raises a "missing field" error because it considers one field at a time. Easy solution is to remove the missing field check. What do you think @jwallwork23 @acse-ej321? :) I removed the test here as it will be removed by #176 (will merge this later).

[ddundo]

Converted to a draft - I want to rethink this a bit

[ddundo]

@jwallwork23 @acse-ej321 ready for review! Sorry for the confusion.

I think this is a much better solution now since the checks are only done once and they are done in MeshSeq.__init__, so any errors will be caught immediately. Currently errors in get_form would be caught at the very end of indicate_errors, after all forward and adjoint solves - so it's a significant waste of time.