[feature] : Surface RMS roughness metric

[MaxGamill-Sheffield]

Is your feature request related to a problem?

It would be nice if an RMS roughness metric was obtained for the background of each image.

Describe the solution you would like.

The filters stage obtains a background only mask (inverse of foreground mask) which could be used to obtain this using some code @SylviaWhittle has previously written. This could be appended to the image_stats output.

Describe the alternatives you have considered.

No response

Additional context

No response

[SylviaWhittle]

I just remembered - we already have RMS roughness for the whole image, was written a while back. Adding background roughness will just be a case of passing a masked array (and possibly updating the function to accept masked arrays), roughness_rms function from TopoStats.statistics