Move constants to context-managed GridfinityConfig

[cgevans]

In order to allow objects with altered dimensions (eg, a unit other than 42 mm, or altered tolerance), this commit moves constants into a GridfinityConfig class. These can be altered through either passing an instance to the GridfinityObject constructor, or using the class as a context manager, for example:

# Creates a box and baseplate with a 20mm width unit and 5mm height unit.
with GridfinityConfig(GRU=20, GRHU=5):
    box = GridfinityBox(2, 1, 3)
    baseplate = GridfinityBaseplate(4, 3)

# This uses normal sizes
baseplate_normal = GridfinityBaseplate(4, 3)

I need to add tests and documentation.