Add constructor for BandedMatrix from AbstractSparseMatrix (or SparseMatrixCSC)?

[devon-research]

Seems like it would be useful.

(Example: some packages use sparse matrices in their implementation internals and/or output sparse matrices; with this constructor one could easily take the output and create a BandedMatrix to then use for further computations.)

findnz for the sparse matrix would probably make it straightforward.

[dlfivefifty]

Sure! A PR would be appreciated