Refactor of observe_process

[athowes]

observe_process creates columns ptime_daily and stime_daily. This seems to be strange to me. What if the columns are already daily (e.g. #52 (comment))? What if we want something other than daily level of censoring? This suggests function refactoring to me.

[seabbs]

The current approach is 100% hardcoded for the original paper we need to think carefully about the preprocessing i.e should this function even exist and if it does in what form/how exposed to the user (i.e is it in post on a model object or preprocessing)

[athowes]

Dupe of #33