Add k'th starting superdiagonal argument to LowerTriangular and UpperTriangular

[cossio]

tril(M, k) and triu(M, k) have an argument k controlling the starting superdiagonal index.

Currently LowerTriangular and UpperTriangular (which return views instead of copies) do not offer this option, and can only return triangular matrices that start at the main diagonal.

Consider adding an option k controlling starting diagonal index also for LowerTriangular and UpperTriangular.

[stevengj]

The main reason for the LowerTriangular and UpperTriangular types is to support special methods that only work for triangular matrices. Special matrix types for $k \ne 0$ would need to be justified by the existence of special algorithms for those cases.

For example, triu(M, -1) corresponds to an upper-Hessenberg matrix, and we have an UpperHessenberg type to support this important case (which arises in a number of applications). However, the algorithms for UpperHessenberg are almost entirely different from those for UpperTriangular, so there doesn't seem to be any advantage in making them special cases of the same type.

[jishnub]

Perhaps BandedMatrices.jl might be suitable for this, as it supports matrices with arbitrary bandwidths