horzvert_timeseries.py: should support 2 mask-thresh arguments  plus other modifications

- --mask-thresh: default should be no masking (use existing masks). It should accept one or two arguments. If two then different values for the two data sets will be used.

--overwrite. will remove after AGU or rename
--geom-file. Can probably be removed. unclear why ascdesc_2_horzvert.py has it. It should find it if it is there,


