Extend SparseMatrixCSC interface (nzrange - rowvals - nonzeros) to views of sparse matrices

I want to extend the nzrange - rowvals - nonzeros interface from SparseMatrixCSC to views of sparse matrices. That would allow to re-formulate many of the standard algorithms for SparseMatrixCSC and extend them to the views without further effort.

The existing exported functions SparseArrays.nzrange … are currently only defined for SparseMatrixCSC. Most existing algorithms for SparseMatrixCSC access the data fields colptr, rowval and nzval of SparseMatrixCSC directly. It would be easy to replace the access by calls to the interface functions. The implementations of these is trivial for SparseMatrixCSC. For a certain class of views of SparseMatrixCSC it is also possible to define the functions in a way, that makes the re-written algorithms working for the views as well. I think, that was the original idea for defining the interface functions, which was not completed yet.

At the same time, some unexported functions have been defined to access the fields (getcolptr, getrowval, getnzval), which are essentially equivalent to nzrange, rowvals, nonzeros, which could be deprecated.