LCD Display: Display Energy formatted to significant figures instead of decimal digits.

[rbsexton]

The existing FW scales energy to SI notation and then places one digit after the decimal point.

This doesn't work so well for low values in kWh, ie 1.3kWh.

The ADCs in the system deliver about 3 significant figures, so display this number to a total of four - ie:

1.305kWh
13.05kWh
130.5kWh

[jeremypoulter]

@rbsexton thanks for the contribution, been something I have been meaning to do this for a while. Will check it over.