Added code coverage in CI

[4c3y]

Features

Code Coverage

• Added code coverage for unittests
• Added job to print code coverage
• Upload coverage report to github pages (Only on master branch)
• Added link to most recent coverage report in README.md
• Added ENABLE_COVERAGE switch in CMakeLists.txt

To suppress some warnings the -fcompare-debug-second compiler flag is used in lpp. This can not be used in combination with the -ftest-coverage to generate a coverage report because some .gcno which are required to generate the coverage report are not generated.

From the GCC docs (https://gcc.gnu.org/onlinedocs/gcc-9.4.0/gcc/Developer-Options.html):

This option is implicitly passed to the compiler for the second compilation requested by -fcompare-debug, along with options to silence warnings, and omitting other options that would cause the compiler to produce output to files or to standard output as a side effect

(This took me only half a day to figure out 😅)

Other

• Added support for gcc-11
• Added CI badge in README.md

[michaelpantic]

LGTM, nice!