* implement the corner() API change: new methods topLeftCorner() etc

* get rid of BlockReturnType: it was not needed, and code was not always using it consistently anyway
* add topRows(), leftCols(), bottomRows(), rightCols()
* add corners unit-test covering all of that
* adapt docs, expand "porting from eigen 2 to 3"
* adapt Eigen2Support

unsupported/Eigen/src/NonLinearOptimization/covar.h

@@ -55,7 +55,7 @@ void ei_covar(
     }
 
     /* symmetrize the covariance matrix in r. */
-    r.corner(TopLeft,n,n).template triangularView<StrictlyUpper>() = r.corner(TopLeft,n,n).transpose();
+    r.topLeftCorner(n,n).template triangularView<StrictlyUpper>() = r.topLeftCorner(n,n).transpose();
     r.diagonal() = wa;
 }