create custom django fields for working with undate and undate interval in a django database #68

• add optional requirements section for django (should not be installed by default); adjust test matrix to test with and without django
• create and test custom model fields and/or descriptor for undate objects
• basic validation for supported formats

resources:

• https://docs.djangoproject.com/en/4.2/howto/custom-model-fields/
• partial date field and descriptor in Shakespeare and Company Project codebase: https://github.com/Princeton-CDH/mep-django/blob/main/mep/accounts/partial_date.py