added publishers for covariances
-added DaoubleMatrixStamped message
-publishers for core state covariance, aux state covariance, and core-aux cross-cov
-extended state definition and counters for core error state counting
fixed doc
fixed matrix block reference issue and corrected formatting
further formatting and row-major storage comment
Merge pull request #36 from ethz-asl/feature/cov_publishers

added covariance publishers