Improve Signaletic's clock detection algorithms

[colinbdclark]

Currently, Signaletic's sig_dsp_ClockFreqDetector (and the Daisy-based hardware examples that use it) is not particularly accurate, largely because of the lowpass filtering done to incoming clock signals.

After reading more about the problem, it seems simpler is better. Most simply use two clock pulses in a row to calculate tempo, with no smoothing or averaging. Sometimes an exponential moving average of multiple clock pulses can be useful (when tempo is tapped manual) but in practice a simple non-averaging calculation is probably best.

[colinbdclark]

Fixed in #68.