[Rivet] Moving plotting metadata out of source files

[Frank Siegert]

Hi Riveters,

I have committed a change to trunk today, which moves the plotting 
metadata (like title, xlabel, ylabel) out of the analysis source files and 
into plain text files in data/plotinfo/.
The reasoning behind it (in addition to code cosmetics) is the wish for 
optionally setting more plot related information, like logarithmic axes 
on/off, legend position, ..., which would be too cumbersome to drag 
through the source code.

So whenever you come across a plot now that needs some additional love to 
look presentable, please take the time to add the necessary information 
(in make-plots syntax) to the respective file in data/plotinfo. I have 
already migrated all title/xlabel/ylabel information from the .cc files.

The only place where this information gets used at the moment is compare-
histos, but in the long term (i.e. when YODA arrives) we want to make it 
programmatically accessible as well.

Hope you find it useful, and please let me know if you notice anything 
strange or broken.

Cheers,
Frank