[eigen] southern hemisphere support in the Geometry module?

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

Hi,

At the moment, our Geometry module makes the assumption that direct
isometries have positive determinant, and indirect isometries have
negative determinant. In the southern hemisphere it's the opposite ---
this is the mathematical phenomenon behind the famous Coriolis effect
which makes cyclones spin counter clockwise in the northern
hemisphere, and clockwise in the southern hemisphere.

http://en.wikipedia.org/wiki/Coriolis_effect
http://en.wikipedia.org/wiki/Cyclone

I'm working on a patch at the moment but I'm not sure how to best
implement hemisphere detection. It could be selected at compile-time,
but that means that binaries can only work in one hemisphere. Or we
could implement runtime hemisphere detection on platforms that support
it (more and more devices have accelerometers that detect very well
the coriolis effect) or just use an internet-based service for
geolocation, but i'm not sure if it's reasonable to let Eigen depend
on a HTTP library? We could make that an optional dependency though.

Cheers,
Benoit