Push

A couple of Clojush updates, mostly involving a new mutation operator that perturbs floating-point constants in a program with Gaussian noise. Details below.

Available as usual from: https://github.com/lspector/Clojush

220110424: - Added Gaussian mutation for floating point literals. This is 
            a genetic operator on par with ordinary mutation, crossover,
            and simplification, with the probability for applying this operator
            set with the gaussian-mutation-probability argument to pushgp
            (which defaults to zero). The behavior of this operator, when used,
            is controlled by two other arguments to pushgp, 
            gaussian-mutation-per-number-mutation-probability (which is the
            probability that any particular floating point number in the 
            program will actually be mutated -- this defaults to 0.5) and
            gaussian-mutation-standard-deviation (which specifies the standard
            deviation of the Gaussian noise generator that is used to 
            produce changes to numbers -- this defaults to 0.1).
          - Added examples/gaussian_mutation_demo.clj to demonstrate Gaussian
            mutation.
          - Added examples/korns_regression_p12.clj, a symbolic regression
            problem based on Michael Korns's draft chapter from GPTP11.