Adding column iterators

[AtheMathmo]

We have added row iterators with #48, #49 . These were easy to do because our matrices are row-major. as mentioned in #48 it would be nice to have column iterators too.

This ticket is to track the implementation of column iterators. The first step should be to write up a specification for how exactly these should look.

[AtheMathmo]

One way to do this would be to have the iterator produce MatrixSlices. This would mean setting the column size of each MatrixSlice to 1 and letting the row_stride be the actual row_stride.

Advantages:

We do not need to copy any data. This is left to the user if they want to use contiguous blocks.

Disadvantages:

Differs from the row iterators which return slices. We opted to return slices in that case to assure the user that the data was indeed contiguous (which provides optimization benefits). If we were to modify this to return MatrixSlices we would not be able to guarantee this so strongly.

[AtheMathmo]

This issue was moved to AtheMathmo/rulinalg#3