[eigen] block of dense-block should again be block

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   I have a dense matrix(expression). When I create a block out of it and 
another block out of this block, I should get an object of type 
Block<some-DenseBase-derivative>. Currently, I get 
Block<Block<some-DenseBase-derivative> >. Studying the code, this is the 
natural way to cover all cases but can the block-of-block-of-X be collapsed 
to just block-of-X when X is dense? AFAIK this should also apply to all X 
besides dense (please correct me).

  I found this out the hard way. the block-of-block is not obvious till a user 
reads the code - I kept assigning this to a simple block as follows

   MatrixXcd X;
   Block<MatrixXcd> const X_block = X.block().block();

and I kept getting seg-faults because X_block has a new return value for 
data() when compared to X (I found this out in the debugger). Correct code 
should look like,

    Block<Block<MatrixXcd> > const X_block_block = X.block().block();

   I previously used Boost::UBLAS where they had free subrange() and project() 
functions which were associative in the above sense. This associativity would 
be a nice addition to Eigen3.

   If the associativity (endomorphism on the space of data-types, actually) 
holds for all types X, then can a member function be introduced into class 
Block that solves this problem? I thought I would try this and send a patch 
but I am not sure the endomorphism will hold for all expression types X that 
Block<> could apply to.

   As a user I suggest that the documentation should warn users about this 
case if this feature is not implemented.


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