Corrected a bug that caused exception when multiplying large amounts of
totalFixedCost

Also changed the way the cost center types are retrieved, now we leverage
on CostCenter::getTypeChoices() instead of build an array ad-hoc
Modified getTypeChoices() so that it now returns an associative array
Added mailer_default_invoice_recipient parameter
Fixed the behaviour of the recipients auxiliary file in the form

It now displays properly the actual recipients of the invoices and the
default recipient if none is found
Usage of CostCenter::getTypeChoices() and mailer_default_invoice_recipient