Remove non-batch mode

Currently, I don't see a need (for my needs) in a non-batch mode, so this
change makes the code less and better to maintain.
Make BIC a required configuration field
Make payment validation more robust

This change adds a check for the existence of all required payment
fields.
Add a payment check to prevent negative amounts
Make _add_to_batch_list shorter
Change file format version to pain.008.003.02