Always include the fittest individual in the next generation.  Sort
individuals fittest-first.  When computing the crossovers, use only
the first result (for GA it seems more usual to include BOTH offspring
from a cut-and-splice or one-point-crossover, but currently our tree
crossover for GP returns only one value.  Perhaps that should change!
To research...)
Included a simple regression problem for GP.  Doesn't converge on a
solution every time...  sometimes does so quickly.
Tweaked