Unfinished therapies

[peterdutey]

Prescriptions with 'active' status (still open) may not have a stop date. Most systems allow to keep prescriptions open indefinitely.
Currently validate_prescriptions() requires that a prescription_end value is provided in order to avoid problems with the therapy episode creation.
The current way for users to work around this is to enforce an arbitrary prescription end date, or to exclude unfinished prescriptions from the dataset.

• This should be documented in validate_prescriptions

In the future there could be an enhancement to support active prescriptions with no end date.

• test and if necessary improve the union find/transitive closure to work with prescriptions with no end date. The therapy_end value should then become NA/NULL
• work out a way to build a therapy table for a therapy with no end date.