- likely also a performance improvement

Fixes error in debian python3-glue install hook:

  File "/usr/lib/python3/dist-packages/glue/dagfile.py", line 946
    code = 'digraph "%s" {\nnode [color="%s", href="\N"];\ngraph [bgcolor="%s"];\n' % (title, colour, bgcolour)
            ^
SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 40-41: malformed \N character escape

Fixes error in debian python3-glue install hook:

Sorry: TabError: inconsistent use of tabs and spaces in indentation (cbcwebpage.py, line 170)

Fixes error in debian python3-glue install hook:

Sorry: TabError: inconsistent use of tabs and spaces in indentation (cbcwebpage.py, line 152)

Fixes error in debian python3-glue install hook:

Sorry: TabError: inconsistent use of tabs and spaces in indentation (query_engine.py, line 113)

Fixes error in debian python3-glue install hook:

Sorry: TabError: inconsistent use of tabs and spaces in indentation (segmentdb_utils.py, line 90)

- to reflect current behaviour

- helps ensure the document contents are stable, which helps the unit tests pass
- (unit tests are based on comparing a test output to a pre-recorded output)

- defer assignment of table IDs until rows are inserted to ensure the IDs are assigned in the order of the rows in the table.  makes the document prettier, and helps ensure the document contents are stable to help the unit tests pass

- refs #4716

- import of glue.ligolw.table no longer required by example

- treat CacheEntry objects that differ only by URL as equivalent.  teaches
  this code the concept of redundant fail-over copies.

- reduce code duplication in handling of GPS and segment-valued properties
- expand the docs to increase test coverage of GMST assignment during GPS time assignment

- to cover support for infinite segment bounds, intervals of validity

- the .comment attribute of SegmentSum rows was failing to be set to None

- gives SegmentSum all of Segment's super powers
- can be used in segment arithmetic, supports infinite-valued boundaries, etc., etc.

- makes it easier to see what they are and understand edits