Add scale parameter to topology graphs

[lukasturcani]

It would be nice if the user could fine-tune the scale of a topology graph on a per instance basis. For example

import stk

bb = stk.BuildingBlock('BrCCBr', [stk.BromoFactory()])
polymer = stk.ConstructedMolecule(
    topology_graph=stk.polymer.Linear(
        building_blocks=(bb, ),
        repeating_unit='A',
        num_repeating_units=2,
        scale=0.5,
    ),
)