Separate energy counters for IN/OUT

[magnayn]

There may be another way to do this..

In my emonTX setup, I have solar PV, but I don't have access to clamp the solar production, only at the grid.

Since there are times of day where the energy is flowing out - negative - this is currently reducing the energy count for that channel. Unfortunately it doesn't mean my meter also flows backwards, so it's thus very hard to compare the values. I don't have an export meter, so one of the uses of having the emonTx there is to be able to work out what proportion is being used on-site and what is being exported.

I've modified it to split the wh out and in. It might be better to wrap the wh_ and residual_energy, and even the channel_in_use in a struct. I retained the old EmonLibCM_getWattHour API for compatibility.