Dual group behavior in Plugin Factory

[teddyrendahl]

Describe the bug

There is a function named group() on the factory but self.group is overwritten in __init__. Seems to work fine, but slightly confusing that this is a method overwritten as an attribute

https://github.com/slaclab/pydm/blob/837f8d546c06cc390e5d05fca418eb9ea552db68/pydm/widgets/qtplugin_base.py#L128

Expected behavior

def group(self):
   return self._group

[hhslepicka]

Solved by #423