* extract_number(), extract_numbers(), and all
   helper functions gain a keyword parameter
   `decimal_places` (for helpers, just `places`)
   which does what it sounds like, using builtin
   round().

 * avoid capturing non-adjacent numbers as decimal
   places

 * avoid capturing already-used decimal places as
   separate numbers in extract_numbers()

 * add a few tests for the above

- more visible funcs now have more explicitly-named parameter for places
- `extract_number(decimal_places=...)` now has several options:
  - `decimal_places=n` will round to `n` places
  - `decimal_places=0` will round up to nearest int, equiv. ceil(result)
  - `decimal_places=-1` will round down to int, equiv. floor(result)
  - expanded comments and docstrings
- remove old commented-out code