Alpha release 0.7.0-alpha

Refactor application to use Database object in definitions.DATABASE for all interactions with persistence.

Added

• Implement Database object to handle persistence
• Implement JSONDatabase(Database) (original/current database backend).

Changed

• Refactor all code/calls dealing with persistence to definitions.DATABASE.
• Student.avatar_filename changed to Student.avatar_id for naming consistency between database backends. This is backwards incompatible, but is a simple string replace operation in any current data files.
• JSONDatabase's Registry now checks if on-disk version of registry is correct, only writing to it if incorrect or non-existent.
• Increased test coverage, more tests converted to Pytest style tests.
• When clicking 'x' instead of 'save as' when a chart is displayed, UI no longer freezes, nor pops up a 'save chart as' file dialogue.

Removed

• class_registry_functions.py, test_class_registry_functions.py: functionality moved to Registry object in persistence/databases/json_registry.py.

Depreciated

• Python 3.6 support ends with this release.
• Python 3.7 support will soon be removed also, in next release following release of python 3.9 - plan is to only support 2 minor releases of python at one time.
• JSONDatabase might be removed at some point, or not support new features, although it, or the data format might be kept for utility of debugging and editing.
• data_version_conversion.py will not be supporting conversion from older formats than current to any future versions.