Add convenience methods for classic inverter power control #142

[istaveren]

Discussion 142 feat: add convenience methods for classic inverter control

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**Summary**

All details in https://github.com/indykoning/PyPi_GrowattServer/discussions/142

**Checklist**

- [x] I've made sure the PR does small incremental changes. (new code additions are difficult to review when, e.g. the entire repository got improved codestyle in the same PR.)
- [x] I've added/updated the relevant docs for code changes i've made.