Generate UID lists

[jas88]

Description

Follow-on to #98 - it would be useful to generate a CSV file with UIDs for use elsewhere. Possibly sets of UID 3-tuples to use as (study,series,instance) UID.

[tznind]

The --csv option sort of does this. It generates CSV files with a given set of headers (study/series and image). We could probably adapt that code with a spot of inheritence to output a single file with Study/Series/Image UIDs or just all tags period in a single file.

For example --csv --single-file