Tuning of parameters and enabled option to choose merge method.

spikesort_muxi has been added to custom/ as a new parameter is defined
which determines which merge process is used in get_final_units.
get_sane has some parameter adjustments--most significantly the
loosening of the 'steepness' criteria. get_final_units now uses the
parameter defined in spikesort_muxi to choose a merge process.