Add round-tripping of doses between VTMs/VMP/AMPs

[wardle]

We can already find the VMPs for a VTM, and the AMPs for a VMP and vice versa. However, a VTM with a dose should be mapped to a VMP / AMP automatically. For example, amlodopine 5mg od could be mapped to a VTM with a dose or a VMP with an instruction amlodipine 5mg tablets take one tablet daily.