Limit tree growth to the system recursion limit

[e10e3]

Python's recursion limit is obtained with sys.getrecursionlimit(). In this PR, trees will stop at 20 before the limit, to ensure the calling program has enough headroom.

How many "steps" before the limit should be kept (here 20) is open to discussion.

Fixes #1578

[smastelini]

Thank you so much, @e10e3. I will check the Hoeffding Tree guidelines separately.