Add speed radius angle

[peterdesmet]

Fix #210. @MarcusRowcliffe can you review this one?

Adds individualSpeed to event observations:

Average movement speed of the observed individual identified by individualID. Expressed in meters per second. Required for random encounter modelling (e.g. Rowcliffe et al. 2016).",

@MarcusRowcliffe Rowcliffe et al. 2016 seems like the correct reference for speed, but is it correct to say it applies to "random encounter modelling"?

Adds individualPositionRadius to media observations:

Distance from the camera to the observed individual identified by individualID. Expressed in meters. Required for distance analyses (e.g. Howe et al. 2017) and random encounter modelling (e.g. Rowcliffe et al. 2011).

Adds individualPositionAngle to media observations:

Angular distance from the camera view centerline to the observed individual identified by individualID. Expressed in degrees, with negative values left, 0 straight ahead and positive values right. Required for distance analyses (e.g. Howe et al. 2017) and random encounter modelling (e.g. Rowcliffe et al. 2011).

[MarcusRowcliffe]

@peterdesmet All looks good to me. Thanks