Rename "spacing" arguments to something more sensible

[leouieda]

There are many places (like block_split, BlockReduce, BlockKFold, etc) that take a "spacing" argument but aren't directly related to generating grids. This is because these functions/classes using grid_coordinates internally to generate blocks. It doesn't really make much sense for these to be called "spacing" when they actually mean something like "block_size" instead.

We should:

1. Make a list of all occurrences of this.
2. Rename the arguments to something more sensible.
3. Think if they really need other arguments like "shape" and "adjust" (they probably don't).

This is a breaking change and so should be reserved for Verde 2.0.

[leouieda]

Thanks to @indiauppal for pointing this out!