Tree initialisation breaks when min = max depth

[jmmcd]

If we set min = max depth, rhh and PI_grow refuse to run. But they ought to run ok.

python ponyge.py --min_init_tree_depth 11 --max_init_tree_depth 11

This happens at eg

PonyGE2/src/operators/initialisation.py

Line 177 in cef8fe7

elif not depths:

where depths will be the empty list. We ought to be able to "ramp" from depth 11 to 11 (inclusive).