Add canopy (fractional) cover calculation

[maawoo]

Canopy cover is mentioned in the Readme but not yet implemented. Would be great to also be able to derive this metric. I'm particularly interested in canopy fractional cover to compare with GEDI L2B data. But others might be more interested in other definitions of canopy cover.

From the GEDI L2B ATBD:

[iosefa]

@maawoo yes. Thanks. I had some code for this as a work in progress but never went through and published it because it needed more testing. I will try to dig it up and at least start a draft PR for this.

[maawoo]

closed by #36, thanks!