Move tax handling from line item api to BAO to make it available from…

… apiv4

This implementation has some limitations. I only address one in this PR - removing the rounding - as
the focus of the PR is the move.

The rounding from all save layers was previously removed but it was reverted when it was eroneously believed
to have caused a bug. The bug turned out to be civicrm#18297

CRM/Price/BAO/LineItem.php

@@ -26,7 +26,7 @@ class CRM_Price_BAO_LineItem extends CRM_Price_DAO_LineItem {
    * @param array $params
    *   (reference) an assoc array of name/value pairs.
    *
-   * @return \CRM_Price_DAO_LineItem
+   * @return CRM_Price_BAO_LineItem
    *
    * @throws \CiviCRM_API3_Exception
    * @throws \Exception
@@ -53,6 +53,12 @@ public static function create(&$params) {
       }
     }
 
+    $taxRates = CRM_Core_PseudoConstant::getTaxRates();
+    if (isset($params['financial_type_id'], $params['line_total'], $taxRates[$params['financial_type_id']])) {
+      $taxRate = $taxRates[$params['financial_type_id']];
+      $params['tax_amount'] = ($taxRate / 100) * $params['line_total'];
+    }
+
     $lineItemBAO = new CRM_Price_BAO_LineItem();
     $lineItemBAO->copyValues($params);