Draft: Sample-and-hold motion model

Generally, a sample-and-hold motion model is available in VA but not used.

Especially, for offline auralizations the default motion model with sliding window interpolation is not required and a sample-and-hold approach makes more sense, since motion keys are given for each audio block.

Aim of this MR, is to make sample-and-hold motion available as option and use it per default in case of offline auralization.