BUG: Convert float freqstrs to ints at finer resolution
- [x] closes #8419 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Passing `'0.5min'` as a frequency string should generate 30 second
intervals, rather than five minute intervals. By recursively increasing
resolution until one is found for which the frequency is an integer,
this commit ensures that that's the case for resolutions from days to
microsecond
