Raise error when using split_blocks inconsistently

Raise error if trying to split inconsistent blocks and revisit test cases.

A couple of comments FYI @frza:

*) I have removed the add_group for the 1b example of blademesher. I guess it is not needed but it may have implications on the block naming. *) I did not extend the error checks to isplit,jsplit and ksplit (domain.py). I am afraid that will limit their usability.