Initialize GP objetct
        
        Args:
            fast:   if fast==True initialize gpkronSum gp

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