Automated code coverage

[RobertPincus]

The repository includes some examples/ and a few tests/ exercising various options in the code. These are cobbled together into a makeshift continuous integration that ensures changes to the code don't change the answers (unless the point of reference is also changed).

It would be terrific to add automated code coverage to the continuous integration so we know how much of the code is being exercised, and so new code is always paired with new tests. We are signed up with codecov.io but I have not been able to figure out how to produce the .xml or .json files needed with the Intel compiler (see branch feature-code-coverage), nor have I been able to figure out how to manage coverage using the gnu toolchain when source code and executables are scattered across directories.

We'd welcome help or a PR that showed us how to generate code coverage reports during continuous integration.

[RobertPincus]

Note that the current tests leave many routines and blocks uncovered (see #62).