Shimmer GSR change to alg. to convert from adc val

Change to algorithm to convert from raw ADC values to resistance. New conversion uses linear fit and is much more accurate than the (now removed) polynomial fit, especially at the edges of the ranges. Also added a command, smoothTransition(), to eliminate spikes in resistance value at autorange transitions (previous value is maintained until circuitry has settled).
Shimmer GSR changed conductance variable to float

For greater precision