docs: public static int isValid( DMatrixRMaj cov ) {
          if( !MatrixFeatures_DDRM.isDiagonalPositive(cov) )
              return 1;
  
          if( !MatrixFeatures_DDRM.isSymmetric(cov,TOL) )
              return 2;
  
          if( !MatrixFeatures_DDRM.isPositiveSemidefinite(cov) )
              return 3;
  
          return 0;
      }