Dense matrix naming and structure

[peshence]

I propose we have a round of renaming/documenting things that may be unclear, e.g.

• slices take (first,last) but do not return last - switch to e.g. (start, end)
• come up with a clearer naming for Matrix and BaseMatrix - e.g. AbstractMatrix and NumMatrix, if I understood those correctly?
• consider adding "Trait" at the end of trait names?
• split DataContainer trait and structs
• make sure file names match content (e.g. matrix_slices.rs in fact has .block functions, while .slice is a very different thing)
• on that note maybe rename slices (as in raw data references) to something clearer to avoid confusion with slice in the pythonic slice sense?