builder: merge/refactor census_summary modules #1442
Description
Follow-on to #1440
The builder has two different source files named census_summary.py. One computes the census["census_info"]["summary"] table stored in the tiledbsoma artifacts, and the other computes slightly different statistics shown in the builder's log/report we retain elsewhere (not user-facing).
Having two source files with the same filename is confusing in itself, and there is also the risk/history that they perform slightly different calculations causing confusion when comparing the table and logs.