Updated `Allergy` and `AllergyExclusion` models

- Integrated three allergen fields into `allergen` field
- Added `start_date`, `end_date`, and `notes` fields to `Allergy`
- Added `date` field to `AllergyExclusion`
Added `gestational_age_at_birth ` field to `Demographics`
Fixed a minor bug in `Allergy` model
Updated `Problem` model
Added a validator for `status` field of `Problem` model
Tidied up the code a little
Fixed a bug of the preprocessor hook for the SDMX transform
Merge branch 'refs/heads/develop' into cophr