Establish database schema for pacsanini models

[aachick]

Is your feature request related to a problem? Please describe.
The objective of this task is to implement a database structure in relation to what is described in #46 .

Describe the solution you'd like
The implementation of this should be done using sqlalchemy. Utilities should be put into place in order to create the database as well.

Describe alternatives you've considered

Additional context

[aachick]

The database schema will be created for the patients, studies, series, images, and studies_find tables using SQLAlchemy ORM classes. This should ensure that the code can be easily worked on in the future + be compatible with different drivers.

A sub-module should be created to convert a DICOM file into these ORM models.

A utils module inside the db module should be created to contain utility functions. In the beginning, this will only contain a method for initializing the database. In the future, this could contain functions that can ensure the migration of the db schema is alembic is ever used.

The crud methods will be updated to take consideration of the data being split up across multiple tables. This will probably not be implemented in the most optimal way but should be OK for now.

Implementation side note: because patient age will be stored in the studies table, a new converter will have to be written in order to convert DICOM AgeStrings to integers (the patient age in years).