Expose some additional functions from RationalTime

## Feature Request

New feature

expose some of the timecode conversion functions available in Python and C++

ie 

`is_valid_timecode_rate`
`to_timecode`

and the static lists of 

`dropframe_timecode_rates`
`smpte_timecode_rates`
`valid_timecode_rates`

## Description

Would be nice to have!
