Make LagrangianFunctionSpace.is_continuous return False for P0

IMHO it is quite misleading that LagrangianFunctionSpace.is_continuous currently returns True, when the degree is zero. This MR is bound to change this. The operator generation test still works and any potential impact on form generation cannot be tested due to #48 (closed). Thus, I think we are good here.

added refactoring user experience labels

requested review from @he66coqe

approved this merge request

merged

mentioned in commit ba9cc538