AP_BattMonitor: support 18 cell voltages

Author: rmackay9

libraries/AP_BattMonitor/AP_BattMonitor.h

@@ -22,7 +22,7 @@
 #define AP_BATT_MONITOR_RES_EST_TC_2        0.1f
 
 #if HAL_PROGRAM_SIZE_LIMIT_KB > 1024
-#define AP_BATT_MONITOR_CELLS_MAX           14
+#define AP_BATT_MONITOR_CELLS_MAX           18
 #else
 #define AP_BATT_MONITOR_CELLS_MAX           12
 #endif