Improve our Result classes

[mrossinek]

What should we add?

As things are right now, our Result objects are fairly limited in their usefulness.

The only thing which they are capable of is being formatted as strings to be printed. But also here we have been running into problems. For example our ElectronicStructureResult forcefully requires ParticleNumber, AngularMomentum and Magnetization values to be computed. If either of them is missing, printing of the object will fail.

This needs to be fixed and some other improvements I have in mind are the following:

• refactor them as dataclasses (automatically makes them a lot more useful since they can be converted to and from dictionaries)
• add storing/loading capabilities (with the above, json will be almost trivially supported)
• more fine-grained printing control (?)
• deal with problems like this one:

  qiskit-nature/qiskit_nature/second_q/problems/electronic_structure_result.py

  Lines 99 to 100 in bf3ce82

  	# TODO the fact that this property is computed on the fly breaks the `.combine()`
  	# functionality