Fixed the size of generated Forest.

Limit for a number of trees generated in the scope of
one forest. Since the Forest is represented by a class, then
we need to be careful not to polute constant pool of the class.
Fix for prediction on large trees.

Missing conversion to float caused differences between interpreted
and generated version of trees.

Moreover, I there are now 2 GBM tests generating 5000 trees and
verifying that the generated model can be compiled.
However, there are not passing now because on a small difference
on 7th decimal digit which needs to be tracked.