Implement a summer/winter mode

[robdidu]

Thanks for your great project. I had to switch to your project yesterday, cause my inverter stopped beeing able to limit it's output.
What I had in my implementation was kind of a summer / winter mode.
I got the month number from the time package and decided that in months with less (or little to no) sun, the output should be very conservative, therefore allowing to get max 30 Watts from the grid before regulating, which saves our precious battery power.
In summer, when we have enough sun power, the output should be lean, allowing 30W "waste" to the grid, which avoids paying for small power draws due to power requirement oscillations.

On a higher level, one could use the free openmeteo solar forecast, to set the mode for the next 24h hours or so. the charge state of the battery could also be a factor.