Allow configuration of modbus device address ("unit") and used write command

[M4GNV5]

I got some more time to play around with my RS485-TCP adapter and made it successfully talk to my heatpump.
I had to make some further adjustments to this program in order to be able to make it work:

• My heatpumpt only supports multi-writes not single writes. I added a config option to configure which write mode to use
• The program always talked to modbus address 0x01. I added a config option to specify the device address to talk to.

EDIT: I will have a look at the pep8 and coverage comments and try to fix them.

[pep8speaks]

Hello @M4GNV5! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2024-01-15 09:25:01 UTC

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (c02482f) 96.56% compared to head (a8d32cb) 96.53%.

Files	Patch %	Lines
modbus4mqtt/modbus4mqtt.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
- Coverage   96.56%   96.53%   -0.03%     
==========================================
  Files           6        6              
  Lines        1047     1068      +21     
==========================================
+ Hits         1011     1031      +20     
- Misses         36       37       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[M4GNV5]

The @codecov report seems outdated, but from my perspective this PR should now be ready for review / merge @tjhowse

[tjhowse]

Looks good to me! Amazing work! Thank you very much.