Blurb::
Set probability of a mutation
Description::
All mutation types have a \c mutation_rate, which controls the number of
mutations performed. For \c replace_uniform and all the \c offset_* types,
the number of mutations performed is the product of \c mutation_rate and
\c population_size. For \c bit_random, it's the product of the 
\c mutation_rate, number of design variables, and population size
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