[ready-for-core-team-review]CRM-16189, added function to get previous financial item for line item (#8654)
* CRM-16189, added function to get previous financial item for line item
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* CRM-16189: Improve support for Accrual Method bookkeeping
https://issues.civicrm.org/jira/browse/CRM-16189
* CRM-16189 Added unit test for getPreviousFinancialItem
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* CRM-16189: Improve support for Accrual Method bookkeeping
https://issues.civicrm.org/jira/browse/CRM-16189