class RooSimWSTool::BuildConfig

 Class RooSimWSTool is a tool operating on RooWorkspace objects that
 can clone p.d.f.s into a series of variations that are joined together
 into a RooSimultanous p.d.f.

 The simplest use case to to take a workspace p.d.f as prototype and
 'split' a parameter of that p.d.f into two specialized parameters
 depending on a category in the dataset.

 For example, given a Gaussian
 p.d.f G(x,m,s) we want to construct a G_a(x,m_a,s) and a G_b(x,m_b,s)
 with different mean parameters to be fit to a dataset with observables
 (x,c) where c is a category with states 'a' and 'b'

 Using RooSimWSTool one can create a simultaneous p.d.f from G_a and G_b
 from G with the following command

   RooSimWSTool wst(wspace) ;
   wst.build("G_sim","G",SplitParam("m","c")) ;

 From this simple example one can go to builds of arbitrary complexity
 by specifying multiple SplitParam arguments on multiple parameters
 involving multiple splitting categories. Splits can also be performed
 in the product multiple categories, e.g.

      SplitParam("m","c,d")) ;

 splits parameter m in the product of states of c and d. Another possibility
 is the 'constrained' split which clones the parameter for all but one state
 and insert a formula specialization in a chosen state that evaluates
 to 1 - sum_i(a_i) where a_i are all other specializations. For example,
 given a category c with state "A","B","C","D" the specification

     SplitParamConstrained("m","c","D")

 will result in parameters m_A,m_B,m_C and a formula expression m_D
 that evaluates to (1-(m_A+m_B+m_C)). Constrained split can also be
 specified in product of categories. In that case the name of the
 remainder state follows the syntax "{State1;State2}" where State1 and
 State2 are the state names of the two spitting categories.

 The examples so far deal with a single prototype p.d.f. It is also
 possible to build with multiple prototype p.d.fs by specifying a
 mapping between the prototype to use and the names of states of
 a 'master' splitting category. To specify these configurations
 an intermediate MultiBuildConfig must be composed with all
 the necessary specifications. For example, this code

  RooSimWSTool::MultiBuildConfig mbc("mc") ;
  mbc.addPdf("I","G",SplitParam("m,s","c")) ;
  mbc.addPdf("II,III","F",SplitParam("a","c,d")) ;

 configures a build with two prototype p.d.f.s G and F.
 Prototype G is used for state "I" of master split category
 mc and prototype F is used for states "II" and "III" of
 master split category mc. Furthermore parameters m,s of prototype G are split
 in category c while parameter a of prototype F is split in
 the product of categories c and d. The actual build is then
 performed by passing the build configuration to RooSimWSTool, e.g.

  wst.build("MASTER",mbc) ;

 By default, a specialization is built for each permutation of
 states of the spitting categories that are used. It is possible
 to restrict the building of specialized p.d.f to a subset of states
 by adding a restriction on the number of states to build as follows

  mbc.restrictBuild("c","A,B") ;

 The restrictBuild method can be called multiple times, but at most
 once for each used splitting category. For simple builds with a single
 prototype, restriction can be specified with a Restrict() argument
 on the build command line