Allow users to specify a schema to validate against

[helrond]

Is your feature request related to a problem? Please describe.
DACSspace will ship with a schema for single-level minimum requirement and (maybe) a custom RAC schema. Users may also want to supply their own schema to validate against.

Describe the solution you'd like
Allow users to specify which schema they want to validate against. Ideally we should allow both:

• A pointer to an existing schema that is part of DACSspace (presuming there is more than one), somewhat like we do in rac_schemas: https://github.com/RockefellerArchiveCenter/rac_schemas/blob/base/rac_schemas/__init__.py#L49
• A schema as a file object or a filepath, which would allow users to create their own arbitrary schema.

[helrond]

Default to DACS single-level minimum schema.