Switching to SQLite

[paulsaxe]

Switching the storage to SQLite in order to get both relationships and a native file format for the system.

[lgtm-com[bot]]

This pull request introduces 14 alerts and fixes 4 when merging 12acfcf65f144e1aa5ef3306cb08c4bc8cef97ae into f2970a36278da812c71598d5f7874976da17e497 - view on LGTM.com

new alerts:

• 5 for __eq__ not overridden when adding attributes
• 2 for Unused local variable
• 2 for Wrong number of arguments in a class instantiation
• 1 for Comparison using is when operands support __eq__
• 1 for Special method has incorrect signature
• 1 for Variable defined multiple times
• 1 for Non-standard exception raised in special method
• 1 for __init__ method calls overridden method

fixed alerts:

• 2 for __eq__ not overridden when adding attributes
• 2 for Unnecessary delete statement in function