Add a concept of chunking so that `apply` can split jobs along workers better.

[LTLA]

Current thinking is that we have a few virtual functions:

• One indicating whether a matrix is not chunked, chunked by fixed-width intervals, or chunked by arbitrary ranges.
• One returning the fixed width interval, if relevant.
• Another returning the chunk boundaries somehow.

Then apply can make an educated decision on how to define the boundaries of the parallel worker ranges by rounding to the closest chunk boundary. Primarily useful for file-backed matrices.