Optimize Averager/Aligner coefficient calls

[garrettwrong]

In #538 we have some areas of the code which require basis coefficients in BFSR. Currently it gathers the entire coefficient stack, which is a poor implementation. It is possible to refactor the code to gather just the required coefficients.

There is actually a helper method Averager2D._cls_images. It is used for this purpose elsewhere in the code. Then it becomes a matter of refactoring/orchestrating the coefficient calls.

I intentionally did not optimize this because I feel like BFRSAverager2D should be deleted in the next couple releases. However, if you intend to keep it, this is something to fix.

[garrettwrong]

Closed via #755 and the Image accessor updates.