Added an ei_linspaced_op to create linearly spaced vectors.

Added setLinSpaced/LinSpaced functionality to DenseBase.
Improved vectorized assignment - overcomes MSVC optimization issues.
CwiseNullaryOp is now requiring functors to offer 1D and 2D operators.
Adapted existing functors to the new CwiseNullaryOp requirements.
Added ei_plset to create packages as [a, a+1, ..., a+size].
Added more nullaray unit tests.

doc/snippets/DenseBase_setLinSpaced.cpp

@@ -0,0 +1,3 @@
+VectorXf v;
+v.setLinSpaced(0.5f,1.5f,5).transpose();
+cout << v << endl;