ina219: Interpret signed output register values

[eldondevat]

The current and shunt voltage register values are signed, twos complement format. In my tests interpreting them directly in the way implemented here gave output consistent with other ina219 libraries.

[codecov-commenter]

Codecov Report

Merging #54 (ec1825f) into main (212f060) will increase coverage by 0.1%. The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main     #54     +/-   ##
=======================================
+ Coverage   64.9%   64.9%   +0.1%     
=======================================
  Files         60      60             
  Lines       7066    7066             
=======================================
+ Hits        4583    4589      +6     
+ Misses      2329    2325      -4     
+ Partials     154     152      -2     

Impacted Files	Coverage Δ
ina219/ina219.go	100.0% <100.0%> (ø)
ads1x15/ads1x15.go	85.4% <0.0%> (+2.2%)	:arrow_up:

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