single_cover_value usage

[GregGuerin]

Presently subsets point intercept hits based on a minimum height but should also be able to subset by a MAXIMUM height as a user might want ground cover at a certain height rather than tree cover over a minimum height cut-off (the original case).

Also look into simplifying the arguments in the function call by combining the 2 relating to growth form.

[GregGuerin]

Added optional maximum height so you can calculate any height band. Did not change growth form arguments as it isn't broken and don't want to break user code.