[eigen] yet another deep change wrt storage orders...

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

as if we didn't have enough... ever since the RowMajor fixes from
january, we have quite some instability in the development branch:
there's always a little thing broken somewhere and fixing it results
in something else getting broken. Currently, what's broken is certain
matrix algorithms applied to 1x1 matrices.

To summarize a little discussion that happened today over IRC:

[17:30] <bjacob> the bottom of the problem is that our idea of
vector-specific functions that work seamlessly for row-vecs and
col-vecs, detecting 1xN vs. Nx1, completely fails on the 1x1 case
[17:30] <gael_> yep
[...]
[17:40] <gael_> but cannot the RowMajor/ColMajor mean RowVector
ColumnVector for the vector case ?

With that gael stepped over one of my big taboos so i initially
psychically rejected his idea... then I realized he was right, this
actually simplifies a lot of code we have for vectors, and moreover:

[19:26] <bjacob> gael_: wow, here's a very very good reason for it
too: let's constrain the Options to Matrix, to have the right
RowMajorBit. This prevents having 2 different vector types that differ
only by the (unused) RowMajorBit !!

So i'll start that in a fork and report here...

Benoit