Add basic support for PointValues attached to the points of a PointCloud, development towards #1

[fstwn]

Added two new components for adding and retrieving the PointValues attached to the points of a point cloud. Unforunately, there are a lot of bugs in RhinoCommon at the moment, resulting in a complicated implementation (for now). Once the setter works again, this can be simplified a lot.

For further info on the current bugs see: https://discourse.mcneel.com/t/clearing-color-from-point-clouds-using-rhinocommon-clearcolors-in-c/125985/5?u=efestwin