Better selection of Swift/XRT PC data

[chbrandt]

Improve the selection of data files (event-files and exposure-maps) with the help of Master-table's XRT_EXPO_PC column. Although the Master table used by the pipeline is "garbage cleaned", the exposure time in pc mode must be double checked.

A better selection of the data will improve SDS stability and may help communicate with the user. For example, if we ask for MRK421 observations during 2018 (until May), 65 observations were done in that region/object, but only one in PC mode.