fix bug in analytical integrals over RooHistPdf sub-ranges

- fixed typo: frcode |= ... instead of frcode = ...
- reactivate old behaviour that permits analytical integrations for
  interpolated RooHistPdfs, if the integration range is the full range
  of all the integration variable(s)
- fix a bug with multidimensional integrals (my code was a bit too
  clever)

Signed-off-by: Wouter Verkerke <verkerke@nikhef.nl>
RooRandom: add interface to set RNG, use TRandom::RndmArray for greater speed

Signed-off-by: Wouter Verkerke <verkerke@nikhef.nl>