Leave SqDistAlgorithm's state alone if no inputs

Fixes issue #239, whereby the next_starttime state variable was
incremented even if no input was passed to SqDistAlgorithm.process_one(),
and therefore no processing was performed by SqDistAlgorithm.additive().
Also, modified SqDistAlgorithm.additive() so that the state variables
returned were consistent with the last *input* sample, and NOT with the
last output sample (they could differ if fc input parameter were ever
used).