Fixed Gaussian2D model for cov_matrix input
Added change log entry for #2199
Added regression test for #2199
Changed Gaussian2D model rotation to increase counterclockwise
Updated regression tests for #2199
Updated documentation for Gausian2D model
Updated change log entry for #2199
Updated docs to display the covariance matrix as a matrix
Merge pull request #2199 from larrybradley/2dgauss-covmatrix

Fixed Gaussian2D model when a cov_matrix is input