add code coverage to nightly system

[ibaned]

[wrtobin]

https://cmake.org/Wiki/CTest/Coverage has some discussion of this, but was last updated in 2014 and doesn't seem tremendously straightforward.

https://github.com/bilke/cmake-modules/blob/master/CodeCoverage.cmake seems like it might be a good module to use to include coverage analysis.

[ibaned]

thanks, @tobinw. looking at CodeCoverage.cmake, one issue I have is that coverage is done on a per-test basis, and it would probably be better to use gcov's ability to aggregate data from all test runs into a single coverage report.

In relation to #58, I wonder if we should create a coverage report for master, develop, and proposed merges. I think just having a single report for master that updates nightly after the merge would be sufficient.