Implement matrix row and column views

[demianmnave]

From CML1 created by demianmnave : demianmnave/CML1#3

Row and column views should be derived from readable_vector or writable_vector, depending upon the access requirements of the source matrix. The views need not be high performance except in specific circumstances; e.g. a row_view of a row-major matrix should be fast, but a col_view may not be due to stride.