Add a new `is_data_contiguous` function

[AtheMathmo]

We have discussed around this before - it was most recently suggested in #86 .

I think this is a useful utility function that we could have on the BaseMatrix trait. The default implementation would check that the row stride and column count are the same (I think?). We would override the function for Matrix to always return true.

We could then use this check to improve our iterators and perhaps also a few other areas of code.