Return a float instead of an array of a single float.

Currently, if the total energy is, say, -2.2, array(-2.2) is returned.
Instead, let's simply return the float -2.2. This is in better agreement
with Lammps interactive, and appears to be also in better agreement with
Vasp interactive.

pyiron/sphinx/interactive.py

@@ -69,9 +69,7 @@ def interactive_energy_tot_getter(self):
 
     def interactive_energy_pot_getter(self):
         self._interactive_pipe_write("get energy")
-        return np.array(
-            float(self._interactive_library_read.readline()) * HARTREE_TO_EV
-        )
+        return float(self._interactive_library_read.readline()) * HARTREE_TO_EV
 
     def interactive_forces_getter(self):
         self._interactive_pipe_write("get forces")