*add missing overloads of setZero, etc... that were mentioned in the tutorial

--->they go into Matrix as they resize.
*add isConstant() alias to isApproxToConstant()
*extend unit-test
*change an assert into a static assert

doc/C01_QuickStartGuide.dox

@@ -167,7 +167,7 @@ x = VectorXf::Random(size);
 x.setZero(size);
 x.setOnes(size);
 x.setConstant(size, value);
-x.setIdentity(size);
+N/A
 x.setRandom(size);
 \endcode
   </td>