Add config option to manage output data precision

[bachya]

Describe what the PR does:

Since the past PRs re: unit conversion started producing units at high precisions, this PR introduces the ability to control that precision via --precision.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md with any new documentation.
• Add yourself to AUTHORS.md.

[codecov]

Codecov Report

Merging #347 (f1c35a3) into dev (b3a1799) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev      #347   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           29        29           
  Lines         1524      1530    +6     
=========================================
+ Hits          1524      1530    +6     

Impacted Files	Coverage Δ
ecowitt2mqtt/__main__.py	100.00% <100.00%> (ø)
ecowitt2mqtt/config.py	100.00% <100.00%> (ø)
ecowitt2mqtt/const.py	100.00% <100.00%> (ø)
ecowitt2mqtt/helpers/calculator/__init__.py	100.00% <100.00%> (ø)

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