WIP attempt to get serial and RS485 Modbus-RTU working

[iconnor]

Testing on a Acrel acr10r-d16te but it is not working yet so please don't merge.

[pep8speaks]

Hello @iconnor! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file modbus4mqtt/modbus_interface.py:

Line 18:121: E501 line too long (151 > 120 characters)
Line 130:121: E501 line too long (130 > 120 characters)

Comment last updated at 2021-08-16 03:56:14 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.03%. Comparing base (96da28f) to head (6e20a9a).
⚠️ Report is 83 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
+ Coverage   95.83%   96.03%   +0.19%     
==========================================
  Files           6        6              
  Lines         841      857      +16     
==========================================
+ Hits          806      823      +17     
+ Misses         35       34       -1     

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[iconnor]

Still WIP: I am waiting for Acrel to give me the registry map, but if I get an ABB one first, I will post it in the PR

[iconnor]

I have the manual but the values are not changing despite energy flow: https://swordandstone-my.sharepoint.com/:b:/g/personal/ianconnor_swordandstone_com_au/ETclJIj1jX1GreQjrtyl88YBzintfvcHlodOC6UyEoOODg?e=eFKsUB so still have some work on this one.