Enhanced the library with writing settings to the inverter.

[27ascii]

The library has been extended to allow writing of settings to the inverter. An example of how it can be used to control battery state is included.

type SettingsDataValues structmatches the response structure of the GET /settings

golrackpi/settings.go

Line 17 in bfd875d

type SettingsDataValues struct {

However it is lacking the valuefield required for writing. Therefor I created ModuleSettings suitable for the updates.
https://github.com/27ascii/golrackpi/blob/8e66f85887f14f082a50b14027045251009f2bab/settings.go#L186

[27ascii]

@geschke Let me know if you need further information on this PR.