NFC code tidy up preparatory to CRM-19273

NFC code tidy up preparatory to CRM-19273

As part of reducing https://github.com/civicrm/civicrm-core/pull/10962/files easier to parse this makes some small
code tidy ups
1) renames functions to not use the deprecated convention of leading underscore
2) renames function to getLineItemsToAlter rather than 'to add & update'
 (later changes will cause this array to include 'to_cancel' & 'to_resurrect')
3) changes the array return format for that function to be an array of requiredChanges
4) rename function _getFinancialItemsToRecord to getReverseFinancialItemsToRecord to
reflect the fact it is generating an array of reversals to perform