Add CanopyProfile class

[davidorme]

This continues to bring in a better structured version of the functionality in #296.

This issue is to develop a class based representation of canopy profiles for a set of stems given:

• plant functional traits for each stem,
• a stem size for which to calculate the canopy profile, and
• an array of heights at which to calculate canopy profile variables: relative and actual crown radius, projected crown area and projected leaf area.

This class is fundamental to describing the canopy layers within the Canopy model but is also usable as a user-facing tool to explore canopy shape parameters.