bump_sensors_calibrate() doesn't allow customized sample count

[adamgreen]

The MicroPython library and the 3pi+ 32U4 Arduino library both default the sample count at 50 but allow the user to provide a different value. Should this same functionality be allowed in the C version by either taking a count parameter and treating 0 to mean the default of 50 or have a separate C function that allows for this count to be explicitly set to a different value before calling bump_sensors_calibrate()?