Updates to the GECE code which is now (briefly) tested:

* clear/set interrupts before/after writing to GECEs
* Store colors in byte sizes and pack to GECE structure only when
  writing out to the GECEs so code re-use is more (for later
  restructuring of all RFPixelControl code)
* Tested using the rainbow pattern using the TestRFTransmitter sketch
* Remove unnecessary (and yet unimplemented) constructor from
  GECEPixelControl class