GEOMESA-163 Update TubeSelect tutorial

Update the TubeSelect tutorial to include "geomesa" as the WPS namespace instead of "vec" and use "dtg" as the default index time field instead of "geomesa_index_start_time"
I don't necessarily think we should change this here, but going forward, `datetime` or even a plain `dt` might serve as a better name for this field.  I had never heard of http://en.wikipedia.org/wiki/Date-time_group but that may simply represent a deficiency in my education.  (Grin implied.)