docs: def initGP(self,fast=False):
          """
          Initialize GP objetct
          
          Args:
              fast:   if fast==True initialize gpkronSum gp
          """
          if fast:
              assert self.n_terms==2, 'CVarianceDecomposition: for fast inference number of terms must be == 2'
              assert self.P>1,        'CVarianceDecomposition: for fast inference number of traits must be > 1'
              self.vd.initGPkronSum()
          else:
              self.vd.initGP()
          self.gp=self.vd.getGP()
          self.init=True
          self.fast=fast