Feature: SIMOVDL invalidate read head

During testing with pynamic, PELICAN, PIGEON and DOVELET, I noticed that it would be helpful to invalidate read heads during runtime. The simplest way to implement this in the existing structure was to use the SetDelay interface. If an invalid delay is set, such as negative sample delays or Nan, Inf values for times, the given read head is invalidated/uninitialised. This has the result that when the subsequent valid delay is set, the first read will not be interpolated (from the previous delay), which is useful in the context of dynamic auralizations with diffraction.