Derived module to import bank statements

This module provides a new backend version, that in turns leverages the account_statement_base_import module.

Each chunk becomes a bank statement. Some flexibility is provided through the dedicated configuration fields on the backend model.
For instance, one can decide to cut in chunks of at most 50 lines having a single date, storing that date as the statement's date. 

One can also decide to base the statement's name on lines labels if lots of line share the same label (can have special functional meaning). Not leaving the default name is good for concurrency (no sequence involved).