Something of a check-point release. Nothing too earth-shattering,
but there hasn't been much done on the code lately, so I wanted
to include what's been laying around for a few months.
* Added the ability to change the reference frequency of the
tuning tables, so that one isn't limited to the 12-EDO standard
for the mapping.
* New functions:
* `prime_factor_ratio()`
* `create_ratio_from_primes()`
Date: 2 December 2016