WIP: Throw exception if curvature is not implemented for RelaxedNewton

The problem: If an energy class without curvature is defined and then minimized with RelaxedNewton, it does not raise an error and continues the script. In this way, I thought that my algorithm estimates noise variances but it doesn't.

I do not know how to implement this check properly. Do I really need to import sys for that and use sys.exit()?

nifty/minimization/relaxed_newton.py

@@ -18,6 +18,7 @@
 
 from .descent_minimizer import DescentMinimizer
 from .line_search_strong_wolfe import LineSearchStrongWolfe
+import sys
 
 
 class RelaxedNewton(DescentMinimizer):
@@ -47,4 +48,11 @@ class RelaxedNewton(DescentMinimizer):
            Returns the descent direction with proposed step length. In a
            quadratic potential this corresponds to the optimal step.
         """
-        return -energy.curvature.inverse_times(energy.gradient)
+        try:
+            curv = energy.curvature
+        except NotImplementedError:
+            print('Error in Relaxed Newton: Curvature is not implemented.')
+            print('Implement curvature or use minimization procedure which uses only the gradient for minimization.')
+            sys.exit()
+
+        return -curv.inverse_times(energy.gradient)