This introduces two new class: BaseMutator and BaseAppStateMutator.
BaseMutator is the base class for all mutators. It stores a couple of

attributes used in the mutator process (can_simulate and finalized),

introduces a function for finalizing, and a default to_sql()

implementation.
BaseAppStateMutator subclasses this, takes the parent AppMutator,

and sets up some forwarding properties that reflect or update the

AppMutator. It contains run_mutation() and run_simulation()

functions as well.
Right now, only ModelMutator subclasses BaseAppStateMutator, but

another will be coming soon.

All unit tests pass.
Tested along with an upcoming new mutator.