Ramped-half-and-half: full and grown trees

[emgrasmeder]

The enhancement on the table is to use Koza's "ramped half and half" algorithm to create trees.

• A "full" tree is a tree such that every path has a depth equal to the maximum tree depth
• A "grown" tree is a tree such that every path has a depth less than or equal to the maximum depth of a tree
• Koza's ramped-half-and-half method produces a set of graphs with a uniform distribution of maximum depths, and for each max-depth, a uniform number of graphs are "full" and "grown".