Added Ornstein-Uhlenbeck entities.

The OU entity implements a stationary Ornstein-Uhlenbeck (OU)
process. The OUNonStationary entity implements a non-stationary
OU process, with mean and standard deviation specified by two
input waveforms. The autocorrelation time constant is fixed.
Work on OU entities and changes to sinusoids protocol.

The sinusoids protocol can now correctly use filtered rates for the
background conductances.