Removing Constant domains everywhere I can find them

[ScottMacLachlan]

In #73 , we removed Constant for all of our time and dt objects, by introducing MeshConstant in irksome/tools.py. This finishes(?) that job, by removing other places where we assigning constants with domains, and using MeshConstant for all of these

[rckirby]

I think this is fine. Do we understand what needs to be a Constant versus having an attached mesh? Is it anything that might get recycled (which a butcher tableau might if you had a loop over stepping methods)?