Changed gate.high_low() defaults.
Changed gate.high_low() defaults.

Changed default high and low values for gate.high_low() to np.Inf and -np.Inf
respectively so that data passes through unchanged if high and low values
aren't specified (previous behavior was to gate out everything because high
and low would remain None).

Fixes castillohair/fc#87.

taborlab/FlowCal,taborlab/FlowCal