Enable additional order-total discounts and/or surcharges

[lat9]

A site- (or plugin-) specific /admin/includes/extra_datafiles can be used to 'register' additional discounts or surcharges provided by other order-total modules. Note that since the extra_datafiles are loaded in file-system order, there's no guarantee of ordering!

Usage:

<?php
// -----
// Register additional order-totals that provide discounts.  Start by initializing to an empty
// array if the variable's not already present.
//
$stats_sales_reports_discounts = $stats_sales_reports_discounts ?? [];
$stats_sales_reports_discounts[] = 'ot_loyalty_discount';

// -----
// Register additional order-totals that provide surcharges.  Start by initializing to an empty
// array if the variable's not already present.
//
$stats_sales_reports_surcharges = $stats_sales_reports_surcharges ?? [];
$stats_sales_reports_surcharges[] = 'ot_insurance';

This usage will enable additional discount/surcharge modules to be added without affecting the base plugin.