Add MixedSet type
Add unit tests for MixedSet
Add MixedDataSet type
Unit tests for MixedDataSet
Add MixedDat type
Unit tests for MixedDat
Add MixedMap type
Unit tests for MixedMap
Add mixed types to sphinx user docs
Build a sparsity block for each pair of DataSets in the mixed case
Implement Sparsity __getitem__, shape, __iter__
More Sparsity unit tests, mostly for mixed case
Add unit test creating a Mat from a mixed Sparsity
Add unit test building mixed sparsity
Create a PETSc MatNest if Mat is built from mixed sparsity
Create PETSc VecNest from MixedDat in petsc_base
Allow accessing PETSc Mat blocks via [] index or iterator
Flatten mixed Args in LazyComputation init and eval

Dependencies for lazy evaluation need to be the constituent Dats /
Mats for mixed types, since a Dat can be part of more than one
MixedDat etc.
Make data for all components of a MixedDat read-only in ParLoop
Refactor host code gen: move itset loop body to separate template