Initial work on simultaneous fits in pT-eta bins.
Add AsymGaussExp PDF shape and use it.

Currently used only as the signal shape as the bifurcated Gaussian looks to fit the background and secondary samples well.
Add Bukin shape.
Tidy up custom PDF importing.
Add custom RooAsymGaussExp shape.
Add custom SmearedLnChiSquared shape.
Rename Ln to Log.
Add RooSmearedLogChiSquared C++ source files.
Remove Bukin PDF.
Tidy up parameter plotting code.
Merge branch 'ipchi2_shapes' into fitting
Rename pseudorapidity -> rapidity and eta -> y.