[eigen] TopicCustomizingEigen

[Bob Carpenter <carp@xxxxxxxxxxx>]

As I was extending Eigen for automatic differentiation,
I was led to NumTraits through error messages and eventually
found your customizing Eigen topic:

http://eigen.tuxfamily.org/api/TopicCustomizingEigen.html

The example looks to be a bit out of date, as the
NumTraits example is missing the two enums:

RequireInitization
HasFloatingPoint

and doesn't mention the four methods I implemented,
epsilon(), dummy_precision(), highest(), and lowest().

I'm also not sure I'm using the Nested typedef properly -- I
just followed your instructions in the doxygen doc to set it
to the same as the template variable T.

The doxygen page for NumTraits:

http://eigen.tuxfamily.org/dox/structEigen_1_1NumTraits.html

is missing the "generated from the following file", which is odd,
as NumTraits.h is where I expected to find it.

- Bob Carpenter
  Columbia Uni, Dept. of Statistics