generalise PDFs in SigmaVar

[ewanwm]

In samplePDFBase we assume that getPDF returns TH1, which is nice and general

MaCh3/samplePDF/samplePDFBase.h

Lines 66 to 67 in ca66648

	virtual TH1* getPDF(const int Selection){ (void) Selection; throw MaCh3Exception(__FILE__ , __LINE__ , "Not implemented");}
	virtual inline TH1* getPDFMode(const int Selection, const int Mode) {

but then in sigma variations it is limited to only TH2Poly

MaCh3/mcmc/FitterBase.cpp

Line 1167 in ca66648

std::unique_ptr<TH2Poly> currSamp(static_cast<TH2Poly*>(samples[ivs]->getPDF(k)->Clone()));

shouldn't be too difficult to make this more general since, just need to add some runtime checks on the num of dimensions and variables of the sample to the sigma var app and set titles etc. accordingly