TOML cofiguration for specifying banks' CSV statement format

[adnilsson]

While the old way works, it was unintuitive and a bit backward in some ways. For example, you needed to specify aliases for the whole header, not just the relevant columns. This is no longer the case after this PR. You could also argue that we have improved separation of concerns now since some validation has moved from converter.py to config.py.

Main feature

Define banks via TOML configuration files (placed in the banks/ directory).

• Use tomli for reading TOML files
  • This means that we are no longer a pure Python project
• Load TOML contents to a class BankConfig that does some validation and normilization

TODO

• [x] Update README
• [x] Make BankConfig's column attributes into properties and move normilization into the getters