Added aggregate config option to Billing 95th percentile calculations (#10202)

* Added configuration options to aggregate input and output bits before making 95th percentile billing calculations

* Changed aggregate to per-bill instead of global.  Added config options for making aggregate the default selected option.  Refactored out mres() calls in touched files.  Changed to Config::get where appropriate.

* Fixed documentation typo

* Fixed scope of aggregate default config option to be under billing

doc/Extensions/Billing-Module.md

@@ -23,7 +23,7 @@ Edit `/etc/cron.d/librenms` and add the following:
 
 Create billing graphs as required.
 
-## Options
+## Data Retention
 
 Billing data is stored in the MySQL database, and you may wish to purge the detailed 
 stats for old data (per-month totals will always be kept).  To enable this, add the 
@@ -36,3 +36,21 @@ $config['billing_data_purge'] = 12;     // Number of months to retain
 Data for the last complete billing cycle will always be retained - only data older than
 this by the configured number of months will be removed.  This task is performed in the
 daily cleanup tasks.
+
+## 95th Percentile Calculation
+
+For 95th Percentile billing, the default behavior is to use the highest of the input 
+or output 95th Percentile calculation.
+
+To instead use the combined total of inout + output to derive the 95th percentile,
+This can be changed on a per bill basis by setting 95th Calculation to "Aggregate".
+
+To change the default option to Aggregate,
+add the following the `config.php`:
+
+```php
+$config['billing']['95th_default_agg'] = 1;  // Set aggregate 95th as default
+```
+
+This configuration setting is cosmetic and only changes the default selected option 
+when adding a new bill