Add mutation potency and frequencys to mutation function

[GitToby]

when calling mutate() there should be an option to mutate a member with a given frequency and potency; defaulting to 1,1.

• Frequency defines on average how many times the function mutate() should actually apply a change to the member, the value between 0 and 1
• Potency defines how much of a change should be applied to the mutation. for example a potency of 2 will apply 2 as much of a change to the member