findValidEpochs

  findValidEpochs returns the valid epochs that a time falls within

  Example:
  epochs=findValidEpochs(chanel, time);
  where
     channel is a sigTOOL channel object
     time is a scalar or vector of timestamps
  and
     epochs is a size(time) vector, containing the relevant epoch for each
            timestamp in time. If a timestamp does not fall within an epoch, 
            epochs will contain zero