Add code coverage support

[emgre]

Closes #299.

• Add code coverage using gcov, lcov and genhtml.
• To get the coverage, enable DNP3_COVERAGE CMake option, then build target coverage (with cmake --build . --target coverage). An HTML report will be generated.
• Added integration with Codecov so we get updates about the coverage results. Also added the badges associated with the branches.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (3.0.0@71f6e0f). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             3.0.0    #301   +/-   ##
=======================================
  Coverage         ?   52.9%           
=======================================
  Files            ?     446           
  Lines            ?   17883           
  Branches         ?       0           
=======================================
  Hits             ?    9461           
  Misses           ?    8422           
  Partials         ?       0

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