Re-use rms map

[revoltek]

There are cases (e.g. detection of false positive in inverted maps) that it is necessary to use an external rms map without recomputing it. An alternative would be to specify a different image to use to compute the rms map. Would it be possible to add that option?

[mhardcastle]

The rmsmean_map_filename option allows you to specify external files for these...

[JonahDW]

This is a very useful feature, however it does not seem to appear anywhere in the documentation, I only found out about it through this issue!

The name also threw me for a bit of a loop, suggesting that the order is [rms,mean], while in actuality it is [mean,rms].