Detail for my Mk2_multiLoad_CAT5_4.ino sketch

This is an upgraded version of the original Mk2 sketch for use with my
PCB-based hardware.  It still supports two current sensors, CT1 and CT2.  
CT1 is for monitoring the flow of energy at the supply point; CT2 is 
available for monitoring the flow of current to the primary dump-load.  

With this version, multiple dump-loads are supported, each being 
controlled by its own dedicated IO pin.  To generate sufficient spare
IO pins for this purpose, the RF facility has been removed from this code.
By using the pin-saving hardware option, the 4-digit display is still 
available for use.

The primary load (Load 0) is still controlled from the "trigger" port.  The 
five IO drivers that have been freed up by not using the RF module have been 
re-allocated to control five additional loads.  These signals can be accessed 
at the left-hand side of the J1-5 connector on the PCB.  The uppermost pin is 
for Load 1; the lowermost one is for Load 6.  

For versions 1 to 3, all loads are driven using active-low logic. 
For version 4, the additional loads (Nos 1 - 5) use active-high logic.  When using the 
green PCB, these points can all be accessed alongside a 0V pin (with 0.2" spacing).  

Changes for version _3:

- The 'long' variable, cycleCount, which counted mains cycles since start-up, has
  been removed.  This variable would have eventually overflowed which could have
  caused unpredictable effects.  The related functionality has been re-implemented
  using individual 'int' counters.

Changes for version _4:

- a persistence check for the zero-crossing detection has been added.  This 
is to remove any false detections of zero-crossings.  This effect is seen more 
with some types of transformer than others.

- a mechanism has been added to monitor and display the minimum number
of sample sets that occur each mains cycle.  With a 125us timebase, and three
ADC samples per set, the expected number of sample sets per 20ms mains cycle 
is  20 / (3 * 0.125) = 53.33.   Any value less than 53 would indicate a loss of data.   

- The 5 additional loads are now driven active-high rather than active-low.