Numtxt

Added interval function (1287 -> '21 min 27 sec') for making a number of seconds more readable/understandable.

Updated to include the new si prefixes:
quetta (Q, 10^30), ronna (R, 10^27), ronto (r, 10^-27) and quecto (q, 10^-30).

si prefix news:
https://www.bipm.org/en/-/2022-12-19-si-prefixes
https://phys.org/news/2022-11-earth-ronnagrams-metric-prefixes-voted.html

Switched to standard version naming method (`__version__` instead of VERSION)

Happy fourth birthday numtxt! Here's to four more years! (chant annoyingly)

Restored approx handling of small values.

Fixed si handling of zero, made public three functions previously added (gen10_dict, gen1k_dict, table); added formatting option to approx function.