Invoicing code has been poorly integrated into CiviCRM with much copy & paste,
no following of settings spec and an awful lot of outright hacking.
This is a small step towards moving invoice logic to a centralised place -
specifically in this case the unpacking of the non-standard settings that have been
used. Perhaps once using these functions we can standardise them!
Ideally I'd like to see all the invoicing assigns in a function on this
class, better yet once which mimics the pre or buildForm hook & even
better one day to grow up & be a real hook