DEV: Convert stored_xxx defaults to Factory
DEV: Convert stored_xxx defaults to Factory

Since attr.ib()'s with init=False are defined on the class level, and
not in __init__, it appears that they are instantiated as class-scope
members, not instance-scope members, and thus the use of
attr.Factory is needed in order for each instance of TempVars to
curate its own stored_xxx dictionaries.

bskinn/tempvars,bskinn/tempvars