Expose goodwe grid_export switch

[zlymeda]

This change exposes the grid_export setting in addition to the grid_export_limit. grid_export enables/disables the restriction of the export, while grid_export_limit defines how much can be exported. When grid_export is enabled, the inverter takes the electricity from the grid even if we have enough PV to cover the house consumption (even if the export limit is high). It is beneficial to have grid_export off, and turn it on only in two scenarios:

• PV power is getting close to the reserved output
• electricity price is below 0 and it would cost money to sell the electricity

[mletenay]

There was no activity in past week and it got into conflict with upstream changes anyway, so I took the liberty and implemented this directly. It was being requested for a long time ago, I just was somehow reluctant to do it when service call do the same. Anyway, thanks for the suggestion/code.