Allow unit ID to be in range of 0-255.

[aldas]

Older MODBUS specification limited unit ID (slave id) in range of 0-247). See "Modicon Modbus Protocol Reference Guide PI–MBUS–300 Rev. J" page 19. https://modbus.org/docs/PI_MBUS_300.pdf

In newer spec that limitation is removed.

Fixes #159

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.96%. Comparing base (6704b45) to head (d4a7345).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #160   +/-   ##
=========================================
  Coverage     98.96%   98.96%           
  Complexity      936      936           
=========================================
  Files            73       73           
  Lines          2426     2426           
=========================================
  Hits           2401     2401           
  Misses           25       25           

Flag	Coverage Δ
unittests	98.96% <100.00%> (ø)

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