Build ROOT with ImplicitMT and FFTW3

[lmeyergarcia]

• Enable root's ImplicitMT, which can speed up downstream code execution.
  • Implicit multi-threading allows multi-threaded execution in some root classes (e.g. TH1, TTree and RDataFrame). Note that it must still be enabled during execution by calling ROOT::EnableImplicitMT().
• Enable FFTW3 when building root. This is necessary to perform fast PDF convolutions with RooFFTConvPdf.
• Enable root parallel building. By default, seems to set the number of compilation jobs to the number of available threads. However, this behavior can be controlled with the --cores flag.
• Implement custom RooFit PDF RooPowerLaw, defined as $(x - x_0)^c$. Note that starting with ROOT v6.32, RooFit includes the PDF RooPowerSum, which in principle makes this patch unecessary. However, to shift the threshold with RooPowerSum ones needs to transform the observable with RooFormulaVar or RooLinearVar, which triggers numeric integration and slows down the fit. This is not the case with RooPowerLaw.
• Patch RooHistPdf so that it recognizes RooLinearVar as an object with constant jacobian (based on this patch). This allows RooHistPdf to still use analytical integration when its observables are transformed via RooLinearVar. Note that this (and other improvements) are included starting with ROOT v6.30.