Give example of subclassing MoleculeRecord

[lukasturcani]

A use-case of the EA should be extending it with functionality specific to certain topology graphs. For example, it could be useful to subclass a MoleculeRecord to allow mutation and crossover operations specifically for Lineartopology graphs.

class LinearMoleculeRecord(MoleculeRecord):
    def __init__(
        self,
        first: BuildingBlock,
        second: BuildingBlock,
        third: BuildingBlock,
    ) -> None:
    
        MoleculeRecord.__init__(
            topology_graph=stk.polymer.Linear(
                 building_blocks=(first, second, third),
                 repeating_unit="ABC",
                num_repeating_units=8,
            )
        )
        self._first = first
       self._second = second
        self._third = third

    def get_first(self) -> BuildingBlock:
        return self._first

    def get_second(self) -> BuildingBlock:
        return self._second

    def get_third(self) -> BuildingBlock:
        return self._third


class MyLinearMutator:
    def __init__(self, name: str) -> None:
        self._name = name

    def mutate(self, record: LinearMoleculeRecord) -> MutationRecord:
         # change the first building block to something else
         return MutationRecord(
               molecule_record=LinearMoleculeRecord(
                      first=stk.BuildingBlock('BrCCCCCCBr', [stk.BromoFactory()]),
                      second=record.get_second(),
                      third=record.get_third()
                )  
                mutator_name=self._name,  
            )  
         )

cc: @stevenkbennett