modbus service and entity

[renereimann]

Implementation of a modbus service and entity using the pymodbus module.
This implementation is working with our backout controller.

[wcpettus]

This seems to overlap heavily with the code Robert wrote:
https://github.com/project8/dragonfly/blob/feature/modbus/dripline/extensions/modbus/ethernet_modbus_service.py

It seems to follow a slightly reduced set of functionality, but very closely follows that code. It would help my review to understand the development and application so that we can reconcile the two codes now for prompt release (and yes, it's silly that we've had that code sitting in branch purgatory for nine months)

[renereimann]

I finally found a way to make a proper diff between the two implementations. The issue was that the files lived in different directories and thus were not directly compared against each other.

Looking at the diff there are a series of difference. Most likely the issue why the old version did not work for us was because it was operating on input registers while we need operation on holding registers. However the old code is easily extendable and has some decoding / encoding of the payload in addition.

I will merge these two versions once the device is setup, so I can test the implementation.

[renereimann]

All functionality in here is now implemented in #227 . Thus we can close this pull request.