Implement basic identity covariance

[travissluka]

Implement a simple "dumb" identity background error covariance in our Covariance class,

• similar to what is done here https://github.com/JCSDA-internal/fv3-jedi/blob/develop/src/fv3jedi/ErrorCovariance/ErrorCovariance.cc
• enable the the ErrorCovariance test, it should pass.