[Rivet] Merging in CLHEP replacement vectors and matrices

[Andy Buckley]

Hi everyone,

I've just merged the new vectors and matrices into Rivet. At the moment
Rivet compiles but RivetGun won't build against it, due to a few header
issues: I'll try to get those sorted asap.

I reckoned it was worth getting it out now so you can test it and give
me some feedback. Plus, the merge was enough of a pain that I don't want
any more commits to get introduced in the meantime! Hopefully the
Doxygen documentation for the new vectors etc. will be built by the time
you need to use it: I still have to add a load of Doxy comments, though.

While I'm writing, here's a few style/design points relating to stuff I
noticed when merging the new classes into the projections and analyses:

 * Prefer to make a set of constants for analysis cut values rather than
just hard-coding the cut values into the if statements as literals.
Otherwise it becomes very hard to work out what's going on if you didn't
write it.
 * If not using iterators to run over a collection, use size_t rather
than int or unsigned as the type of the index variable.
 * Only use the triple-slash (///) comments for commentary to appear in
the Doxygen HTML output. This typically means the class and method
documentation comments in the header files and only the @todo notices in
the .cc files. Normal // comments still have their place!
 * Remember that classes can't store references as member variables: a
Foo& _bar member probably doesn't do what you intended!

Cheers,
Andy