[Rivet] Splitting up of some MC_* analyses

[Frank Siegert]

Hi all,

I'm planning to split up some of the MC_* analyses into smaller
logical bits. One example is the MC_WJETS analysis, which combines
basically three different types of analyses:

1. Inclusive W properties (mass, pT, ...)
2. Jet properties (pT, eta, ...)
3. Splitting scales in the kT algorithm (d_k, R_k)

The reasons for splitting them up are twofold: The jet algorithm for
2. and 3. above might want to be chosen differently (e.g. anti-kt for
2. and kt for 3.), and users might not always want the full collection
of all the analyses above.

To help with this change, the MC_JetAnalysis class will also be split
into MC_JetAnalysis and MC_JetSplittings, which would contain the
implementation for 2. and 3. above (and is then also used for Z+jets,
gamma+jets, ...).

Would anybody object to this change? I have committed it for the
basics and the MC_WJETS analysis on the aidarivet branch as an
example. If there are no objections, I would extend that to a few more
analyses and also put it into trunk.

Cheers,
Frank