Limit size of subsequent populations

[henryzord]

Once a tree height is reached, it cannot grow any more.
Prevent the code from behaving like this:

iter: 000 mean: 0.684654 median: 0.679487 max: 0.910256 ET: 291.16sec height: 8 n_nodes: 25 test acc: 0.578947
iter: 001 mean: 0.682795 median: 0.692308 max: 0.910256 ET: 274.74sec height: 8 n_nodes: 25 test acc: 0.578947
iter: 002 mean: 0.690731 median: 0.705128 max: 0.910256 ET: 270.86sec height: 8 n_nodes: 25 test acc: 0.578947
iter: 003 mean: 0.701141 median: 0.717949 max: 0.948718 ET: 275.70sec height: 9 n_nodes: 39 test acc: 0.578947