Bank statement reconcile  - extract domain methods

Hi,

The implementation of the new bank statement reconcile is hardly extensible because the methods are cluttered. This proposals aims to customize the domains used for the reconciliations propositions and the filter of reconcilable move lines. 
It only extracts the part which builds the domain, without altering the public API nor the existing behavior so I think it could land on a stable version.

You will find examples of overrides allowed by this change  as well as more details in the commit messages.

Thanks!