Detail for my Mk2_3phase_RFdatalog_3a.ino sketch

This sketch is intended to run on my "Mk2 3phase" PCB.  Voltage and current are 
repeatedly sampled for each phase in turn.  Every mains cycle, the average powers 
for the three separate phases are combined and used to update an "energy bucket".
This "energy bucket" represents the overall energy state of the premises. 

Any surplus energy is made available to dump-loads.  This sketch supports 
three dump-loads which can be on any phase.  The loads are activated in order 
of priority.  An external switch can be used to select between two pre-set 
priority sequences.

Datalogging is supported.  This records the average power and Vrms voltage on 
each phase every few seconds.   If the RF facility is enabled, this data is 
transmitted by RF. RF is enabled by including the literal definition for  
RF_PRESENT near the top of the sketch.  If this line is commented out, RF is disabled.

The same data can be sent to the Serial interface, but this could potentially disturb
the underlying sampling sequence.   

Changes for version _2:

- a twin-threshold algorithm for energy state management has been adopted
- improved mechanism for controlling multiple loads (faster and more accurate);
- ISR upgraded to prevent a possible timing anomaly
- a performance checking feature has been added to detect any loss of data
- the RF69 RF module is now supported
- the control signals are now active-high to suit the latest 3-phase PCB.
- SWEETZONE_IN_JOULES has been replaced by WORKING_RANGE_IN_JOULES.

Changes for version _3:
 - improvements to the start-up logic.  The start of normal operation is now 
    synchronised with the start of a new mains cycle.
 - reduce the amount of feedback in the Low Pass Filter for removing the DC content
     from the Vsample stream. This resolves an anomaly which has been present since 
     the start of this project.  Although the amount of feedback has previously been 
     excessive, this anomaly has had minimal effect on the system's overall behaviour.
 - The reported power at each of the phases has been inverted. These values are now in 
     line with the Open Energy Monitor convention, whereby import is positive and 
     export is negative.
 *   
 * February 2020: updated to Mk2_3phase_RFdatalog_3a with these changes:
 * - removal of some redundant code in the logic for determining the next load state.
 * 
 *
 *      Robin Emley
 *      www.Mk2PVrouter.co.uk
 */