[REQ] Longitudinal table

[peterdutey]

User requirements

AMS: studying treatment switches
PK/PD studies
adding clinical features
adding medication features #66
need to adjust the start and stop to add time before the entity's own start time. See extend_table_start (Hamish for context?)

Function specification

Slot

longitudinal_table is a slot to objects of class TherapyEpisode and Encounter.
See user request too #112 for Patient

Method

See longitudinal_table() and extend_table_start

Database

Longitudinal tables are handled as temporary tables using the dbplyr interface.
Indices are very important to optimise the clinical feature methods