Detail for my Mk2_RFdatalog_3.ino sketch

This is an upgraded version of the original Mk2 sketch for use with my
PCB-based hardware.  It 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.  

Datalog messages are routinely transmitted by the on-board RF facility.  
By using the pin-saving hardware option, the 4-digit display is still 
available for use.

The _2 release includes various changes since the _1 version:

- The transmitted data now only has two integer fields (no message ID):
  . the power at the supply point in Joules (import is +ve, export is -ve)
  . the diverted energy total for today, in kWh
  (note the change in energy sense to match the OEM convention)

- a REQUIRED_EXPORT_IN_WATTS facility has been added.  This acts like a 
  leak in the energy bucket, therefore reducing the amount of surplus power 
  that can be diverted.  When a negative value is entered, this facility
  acts like a PV Simulator, which can be very helful for test purposes.

- The state of the energy bucket is displayed at the Serial Monitor every 
  second in Joules.

- The RFM12B is no longer sent to sleep between RF trnsmissions, it remains
  awake throughout.  The library call rf12_sendStart() has been replaced by
  rf12_sendNow().  These changes are intended to minimise any disruption
  to the continuous sampling process when RF transmissions are sent.

Changes for version _3:

- For compatibility with other versions of the Mk2 code, the variable cycleCount 
  has been removed.  This variable would have eventually overflowed which 
  could have caused unpredictable effects with other versions of the Mk2 code.