Define how to store 'no growth' results #108
Description
Currently Ramses does not specify how to deal with no growth cultures. Users may well
- not load them into isolates, and this will pass validation but later cause issues with therapy_timeline (no isolate_datetime generates Inf values)
- load them as isolates but the documentation and validation should be more prescriptive as to how to load these (particularly what to include in organism_code)