Initial fitting framework.

Still much todo. Mass fits are OK but IPCHI2 fits will require a
"pre-fit" to MC to get shape parameters.
2D fit in mass and IP chi^2.

This now works, but is very messy as it’s an attempt to get the framework to the stage where the output looks like the unbinned 2010 result.
This means lots of 2010-specific hacks and hardcoded values.
Add example fitting script.

As in the last commit, this is very messy and 2010 specific.
Mainly committing for historical records.
Merge branch 'analysis' into fitting
Just enough to get the fits working again.

With the new 2011/2012-formatted 2010 ntuples.
Still plenty of cleaning up to do.
Clean up duplicate code.
Tidy up IP chi^2 fitting stuff.
Clarify secondary fraction meaning.
Code style.
Fit framework is now fully mode independent.
Tidy up fitting script.
Be more careful setting yields.
More data-like initial IP chi^2 mean values.
Add fix_parameter methods to all modes.
Fit all modes.
Change signal_region to a 2-tuple of limits.

Makes it easier when things that don’t take cut strings need to know the signal region.
Tweak fit starting values.
Tweak LcTopKpi lipchi2 range.
Make sure all other parameters aren’t constant.
Add all plotting stuff.