Add an `AbsorbingRandomWalk` movement mode

[ninbrm]

The AbsorbingRandomWalk (also called spatial absorbing Markov chain or SAMC in the literature) is the movement mode that mathematically underlies the RSP. The RSP is the AbsorbingRW conditional upon arriving. Hence, the survival probability for the RSP is by definition 1. Theta controls the overall mortality or absorbtion rate, thus as theta becomes larger only those random walkers that have followed low cost (least cost in the limit) paths will survive, hence the RSP distribution focusing more on low cost paths as theta increases. The AbsorbingRW is before making it conditional, thus what we end up with is a survival probability from s to t.
Functional habitat: defined from the survival probability.
Betweenness: I am not sure -- I need to check this up. This will not be the RSP betweenness, which is conditional upon arrival
Sensitivity: the powermean sensitivity (without theta, that is theta=1) as the sensitivity analysis for this mode.