[Feat] Make subgroups optional in all imagery modules

[baharmon]

Describe the bug
Subgroups are no longer required by i.group, but are required by i.cluster, i.maxlik, etc.
Now that subgroups are no longer neccessary and are harder to create in the GUI, they should not be required by the other imagery modules.
Suggest keeping subgroup input parameter in relevant imagery modules, but not requiring it.

@marisn Related to #1501 and [GRASS-dev] Management of signature files.

[marisn]

See also https://trac.osgeo.org/grass/ticket/3440