Using UpperTriangular and LowerTriangular arrays (or maybe more generally AbstractArrays?) #451

Related to this discourse thread.

Currently, UpperTriangular, LowerTriangular, UnitUpperTriangular and UnitLowerTriangular arrays don't seem to be fully supported. It would be great if those could also be implemented to work exactly as they would normally if they are used in an ArrayPartition. I have not yet checked for different types of arrays, but perhaps the other special array types (sparse, diagonal, etc. ) would also be nice to add if they don't work yet.