Add support for fractional doses (beyond half measures)

[Selbosh]

The package currently supports expressions with half measures in them, including mixed fractions:

doseminer::replace_numbers(c('half a spoonful', 'three and a half tablets'))
## [1] "0.5 a spoonful" "3.5 tablets" 

But more unusual fractions like thirds, fifths and quarters don't really work because it just kind of adds the denominator:

doseminer::replace_numbers(c('thirty three and a third', 'four fifths'))
## [1] "37" "9" 

The problem is with fixing this (assuming such cases appear often enough in prescriptions to be worth worrying about) without unintentionally breaking the parsing of ordinal words, which look a lot like fractions:

doseminer::replace_numbers(c('every third day', 'every fifth hour'))
## [1] "every 3 day"  "every 5 hour"

However with enough ambition, 'every 1/3 day' could be interpreted as '(every day) / 3' and end up giving the right result, with an appropriate redesign of how intervals are calculated.