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_LinSpaced_seq.cpp

@@ -0,0 +1,2 @@
+cout << VectorXi::LinSpaced(Sequential,7,10,4).transpose() << endl;
+cout << VectorXd::LinSpaced(Sequential,0.0,1.0,5).transpose() << endl;