Metainfo update: Energy, Stress, Forces

Here are a few comments:

1. Having a list of repeatable sections (energies, forces, stress) where the contribution type is identified by a metainfo value has some drawbacks: In order to find a specific value, you have to loop over the entire list. I think a better solution from the perspective of finding information would be to have something like this.

   class Energy(MSection):
       m_def = Section(
           validate=False,
           description="""
           Section describing a type of energy or a contribution to the total energy.
           """
       )
   
       reference = Quantity(
           type=np.dtype(np.float64),
           shape=[],
           unit='joule',
           description='''
           Value of the reference energy to be subtracted from value to obtain a
           code-independent value of the energy.
           ''')
   
       value = Quantity(
           type=np.dtype(np.float64),
           shape=[],
           unit='joule',
           description='''
           Value of the energy identified by energy_kind.
           ''')
   
       value_per_atom = Quantity(
           type=np.dtype(np.float64),
           shape=[],
           unit='joule',
           description='''
           Value of the energy normalized by the total number of atoms in the simulation cell.
           ''')
   
       values_per_atom = Quantity(
           type=np.dtype(np.float64),
           shape=['number_of_atoms'],
           unit='joule',
           description='''
           Value of the atom-resolved energies identified by energy_kind.
           ''')
   
   class EnergyTotal(Energy):
       m_def = Section(
           validate=False,
           description="""
           Contains values for the total energy.
           """
       )
   
   class EnergyKinetic(Energy):
       m_def = Section(
           validate=False,
           description="""
           Contains values for the kinetic energy.
           """
       )
   
   energy_total = SubSection(sub_section=EnergyTotal.m_def)
   energy_kinetic = SubSection(sub_section=EnergyKinetic.m_def)
   ...

   Comparing the process of finding information (in Python, but the same logic applied to Javascript used in GUI):

   1. energy_kinetic = next((x.energy_value for x in energies if x.energy_kind == "kinetic"), None)
   2. energy_kinetic = energy_kinetic.value

   This does not, however, allow easily adding new energy types, which was maybe your original purpose. But I think we should try to optimize the process of information lookups, even if we have to make storing the information a bit harder (=you just need to explicitly create metainfo sections).

2. I'm not sure if we need the atom_-prefix used at least in atom_forces and atom_charges. It would be nice if all force-related values would start with forces_-, the same applies for charges, stresses, and energies. But this is a minor thing.