ENH: Acceleration for sim

Allows simulating higher (or lower) sampling rates using an acceleration factor. Note that it simply reads the fif file faster and doesn't do any sampling rate conversion. This is mostly useful for testing the network throughput and processing in MNE-X.

Also, there is currently no way to set the acceleration in MNE-X. We need to figure out a way to have a RT-server plugin specific configuration dialog (will also be needed for BabyMEG2 etc.).