Write out thermal noise map

[tsalo]

Summary

Related to #864. Writing out the thermal noise map from dwidenoise or patch2self would have two benefits: (1) eventually it could be used to implement a fit/apply structure in QSIPrep, once dwidenoise can ingress a precalculated noise map and (2) it will let us look at how noise varies across the brain in QSIRecon.

[tsalo]

For complex-valued data (with dwidenoise) we'll need to rescale the noise map by sqrt(2). See https://mrtrix.readthedocs.io/en/latest/reference/commands/dwidenoise.html#options.