XYZ_sigma of EmitterSet does not convert as xyz does

[Haydnspass]

EmitterSet features automatic conversion of px and nm for xyz which are derived properties of an internal xyz container. They can also be set via a setter method as

em = EmitterSet(xy_unit='px')
em.xyz_nm = ...
--> em.xy_unit : 'nm'

however the sigma values remain in the original unit. I don't really know how to best handle this, or just issue a warning.

[Haydnspass]

Do not try to save the user from doing it wrong. Keep as is.