Warn against poor performance with very large nside_coverage

[erykoff]

@rgruendl has pointed out that things did not work well when he (as a novice user) had tried nside_coverage=1024. And in fact, this is not surprising!

In my usage I have not found anything greater than nside_coverage>128 to be useful.

I will do a couple of benchmark tests to confirm where things really do start struggling, and add a warning if a higher nside_coverage is specified to protect against these sorts of mistakes.

[fjaviersanchez]

Closed by #103