Clarify band stat functions

[oubiwann]

In band.clj I see you are duplicating the Java API, e.g.:

• compute-band-stats
• compute-statistics

We should take a look at these from a Clojure perspective and give better names ... uncover what's actually being done here. (API functions should be clear and give devs an immediate intuitive understanding of what would happen if called; sadly, GDAL Java failed here :-/).