Eigen CI infrastructure

Eigen's CI infrastructure uses three stages:

1. A checkformat stage to verify MRs satisfy proper formatting style, as defined by clang-format.
2. A build stage to build the unit-tests.
3. A test stage to run the unit-tests.

For merge requests, only a small subset of tests are built/run, and only on a small subset of platforms. This is to reduce our overall testing infrastructure resource usage. In addition, we have nightly jobs that build and run the full suite of tests on most officially supported platforms.