Started to work on lino.modlib.finan.

There should be a more or less hard-coded account type accounts.utils.BankAccounts which is used as filter for the choices to be displaied in Journal.account.

UserLevels (which I’m going to rename to UserRoles one day) and AccountTypes are examples of choicelists where inheritance can be useful. But to make it possible, we first need to get rid of the required choicelist in the __init__.

Updated the docstring of lino.core.choicelists which didn’t pass the doctest.