Cone graph object: make the cone angle a configurable attribute

[DocNessuno]

Use case:
In metrology, a cone plot can be used to represent the vectors for each point taken on a surface by a coordinate measuring machine or similar instrument
At moderate point densities and higher, the overlap between the bases of the cones makes the plot unreadable unless the cone size is dialled all the way down (in turn making the vectors' direction hard to distinguish).

Proposed solution:
Have the cone angle driven by a user-configurable attribute (with set limits) allowing for slimmer or fatter cones accordingly to the user’s needs.
Ideally a cone angle of 0 should be allowed to effectively transform the cone to a line of the same length, but this is a less critical point.