Change the itype or dtype of a mutable matrix

[lambe]

We should give the user the flexibility to change the index or data type of a given matrix. This would be useful for interacting with external libraries (e.g., HSL) where the data types are more restricted, but still let the user build the matrix in a convenient format.

A quick way to do this would be to finish the code in the factory method for the case of creating a matrix from another matrix.