Crossover probability for SubtreeCrossover

[gkronber]

Some genetic programming variants allow a copy operation where neither crossover nor mutation are applied to selected parent trees.

This is helpful for certain problem instances using a multi-tree representation (via MultiEncoding) where we want to prevent that a crossover event causes crossover for all trees.

One way to add this is to add a crossover rate to the general genetic algorithm variations that we are using in HL for GP.

Another and easier way is to implement this in the crossover operator using a crossover probability.

[gkronber]

r17490: add crossover probability for subtree crossover and initialized it to 100%

This change breaks reproduceability of old experiments because we added a call to the random number generator.

[gkronber]

r17491: fix reproducability by calling the PRNG only when probability is < 1

[gkronber]

r17492: adapted unit test to compile with r17490