Merge branch 'visible_likelihood' into 'NIFTy_7'

Make `LikelihoodEnergyOperator` visible to the user

See merge request !652

src/__init__.py

@@ -50,7 +50,8 @@ from .operators.value_inserter import ValueInserter
 from .operators.energy_operators import (
     EnergyOperator, GaussianEnergy, PoissonianEnergy, InverseGammaEnergy,
     BernoulliEnergy, StandardHamiltonian, AveragedEnergy, QuadraticFormOperator,
-    Squared2NormOperator, StudentTEnergy, VariableCovarianceGaussianEnergy)
+    Squared2NormOperator, StudentTEnergy, VariableCovarianceGaussianEnergy,
+    LikelihoodEnergyOperator)
 from .operators.convolution_operators import FuncConvolutionOperator
 from .operators.normal_operators import NormalTransform, LognormalTransform
 from .operators.multifield2vector import Multifield2Vector