Choose dimensions or sizes + strides for user-facing operations

[hunhoffe]

Right now, some operations take dimensions and some operations take sizes and strides.

For clarity, but also some implementation considerations, I think it would make sense to change to sizes/strides.

Some pros for moving to sizes/strides are:

• dimensions requires (at least at the python level) both sizes/strides to be default or both not default. It would be nice for a user to take advantage of default values more easily. If a python wrapper function takes these values separately, then python needs to have hard-coded defaults in order to produced valid dimensions, which is not ideal.
• For dma_bds using the task/chain syntax, each dma_db should have at max 3 sizes and at max 4 strides; this is a condition that is not easy to express clearly when using dimensions
• Personally, I find it easier to read when they are specified separately (but maybe that's just me)

This came to my attention through comments of @fifield and @andrej in #1919

As this would be a change that touches many things, I'm recording outside of the PR for discussion and so it does not become a blocker to that PR.

[andrej]

For dma_bds using the task/chain syntax, each dma_db should have at max 3 sizes and at max 4 strides; this is a condition that is not easy to express clearly when using dimensions

Why only max 3 sizes but 4 strides?

Copying my examples from the review of #1919 on why repeat_count and the higest-dimension size/wrap aren't the same:

Examples

These examples assume a highest-dimension stride of 2.

If the highest-dimension size/wrap is 3, the higest-dimension stride (=iteration step) is 2, and the repeat count is 6, the highest dimension offset added will be

Task repetition:             0   1   2   3   4   5  (repeat count = 6)
Highest-dim offset added:    0   2   4   0   2   4
                                         ^
                                      wrap (=3)

If the highest-dimension wrap matches the repeat count of the task, the iteration step will be added each repetition:

Task repetition:             0   1   2   3   4   5  (repeat count = 6)
Highest-dim offset added:    0   2   4   6   8   10 
                                                     ^
                                                 wrap (=6)

In the other extreme, if the wrap is 1, the iteration step never gets added (BD repeated with no offset).

Task repetition:             0   1   2   3   4   5  (repeat count = 6)
Highest-dim offset added:    0   0   0   0   0   0
                             ^   ^   ^   ^   ^   ^
                             w.  w.  w.  w.  w.  wrap(=1)