Rename region properties

@mortenvp @jpihl I finished the renaming, and the names are better with the "min" and "best" prefices.
I also changed the SIMD dispatcher layer to only change the best_granularity of the stack. This means that the min_granularity and the best_granularity are different values now (if an SIMD class is enabled on the CPU). This actually makes a lot of sense.
But the standalone SIMD classes also have separate min and best properties, and that does not make any sense to me. One alignment() and one granularity() method should be totally enough for these. So I tried to remove the best_* variants, but it turns out that we use the same logic to test these standalone SIMD classes and the full stacks. For example, the test code requires a min_granularity and a best_granularity method for the SIMD class. Fixing this problem in the test code would lead to massive duplication.
Do you think that the current API is acceptable? Do you have any suggestions?
@mortenvp There is a good reason to measure the granularity in value_types: All the region arithmetic function operate on value_type elements, so it would be weird to use bytes here.
An application using fifi can transform this to bytes if needed, but the current approach seems very natural in fifi itself.