Generalize tuple space to accept mixed grid part

With this change it is possible to define a TupleDiscreteFunctionSpace with spaces defined over different grid parts. The only restriction is that they need to have the same EntityType.

The change is backward compatible (the old constructor has a unused warning but this is a false positive).

Now each sub-space is created directly from the associated grid part while the common the firstGridPart is passed to construct the TupleMapper and the DiscreteSpaceDefault. Therefore, iterating over the TupleDiscreteSpace has the same effect of iterating over the first GridPart.