[Use case] Compute geometric parameters of steps

[basille]

Use case: Trajectories are defined as series of steps; inherent to this are the geometric parameters of each step (in addition to geographic and time coordinates): increments in X and Y direction, step length, absolute and relative (turning) angle, net square displacement (NSD) from a reference point.

Requirements:

• Geometric parameters should only be computed on demand: for instance, if subsetting the trajectory or adding missing data (among other examples), the previously computed parameters should remain unchanged. However, recomputation should be allowed.
• It should be possible to compute each parameter independently (i.e. only turning angles or NSD), but also several (or all) at once.

Input: A sftraj object.

Output: A sftraj object with additional columns for each parameter. The names of these columns could be defined by the user with sane standard.

Additional information: Does this depend on projection? Also check creation (as.ltraj) and recomputation (rec) of ltraj objects in adehabitatLT.

[basille]

Closing this issue now that sftrack is developed and on CRAN.