Add support for B+G e-tech WS100/DS100

[Vogelbecker]

Hi,

I would like support in setting up additional electricity meters.
In Germany you can get two RS485-capable devices from B+G e-tech quite cheaply:
1-phase: WS100 ( https://data.xn--stromzhler-v5a.eu/manuals/bg_ws100serie_de.pdf )
3-phase: DS100 ( https://data.xn--stromzhler-v5a.eu/manuals/bg_ds100serie_de.pdf )

I have both models in use (connected to an RS485-to-Ethernet converter).
The corresponding registers can be found in the manual linked above.
Where are the registers added to the template? Then I can test this myself if necessary.

[andig]

Happy to take a PR, see https://github.com/evcc-io/evcc/tree/master/templates/definition/meter for examples. Could also be added to https://github.com/volkszaehler/mbmd/tree/master/meters/rs485 instead.