Allow to modify sub-sections via python fields.

We want to extend the metainfo to allow something like this:

parent.energy_total = Energy(value=1.0)

This would replace:

parent.m_add_sub_section(ParentClass.energy_total, Energy(value=1.0))

This would be possible, but its quite some work to make it work with repeating sub-sections. Here you want something like:

run.section_single_configuration_calculation.append(SCC())

Remember that we need to hook into the process and modify both parent and sub-section. To implement this, python properties are not enough, because for repeating sub-sections we need a special metainfo list class to hook into append (and other modifying methods). Possible, but quite some work.