Cassini ISS timing was missing information in 1999 and 2000

[matthewtiscareno]

As far as we can tell, for all of the Cassini ISS images taken from 1999 to mid-2000, the PDS3 labels have START_TIME and STOP_TIME equal to each other, but the difference between SPACECRAFT_CLOCK_START_TIME and SPACECRAFT_CLOCK_STOP_TIME is 1000x the EXPOSURE_DURATION — that is, an amount that is equal to what the EXPOSURE_DURATION would be if the latter is interpreted as being in units of seconds rather than milliseconds. The PDS4 labels follow this.

We will fix this in PDS4 only. For PDS3, we will add disclaimers in a bunch of places, including an errata file.

For PDS4, we will first verify that stop_date_time corresponds to cassini:spacecraft_clock_stop_count per a known SPICE conversion. We will then subtract cassini:exposure_duration (in the proper units!) from stop_date_time to obtain start_date_time. We will then use the same SPICE conversion to obtain cassini:spacecraft_clock_start_count from start_date_time. We will round cassini:spacecraft_clock_start_count to the nearest whole number.