Add analytical integral for PowLawAcceptance

Uses GSL for hypergeometric function ₂F₁(a,b;c;x)
Temporarily turn down the integral accuracy.

See errors as below:

  [#0] ERROR:Integration --
  RooAdaptiveGaussKronrodIntegrator1D::integral() ERROR: roundoff
  error prevents tolerance from being achieved
Link to GSL properly
Fix GSL include directory
Implement PowLawAcceptance::analyticalIntegral() (temp commit)