populate "Generated by" in EnergyAdjustment generated by MP2020 and fix serialization bug

[rkingsbury]

Populate the cls kwarg when creating EnergyAdjustment within MaterialsProject2020Compatibility. This information will be helpful in tracking the provenance of energy adjustments.

Now that the cls kwarg is populated as intended, executing ComputedEntry.energy_adjustments on an entry that has been processed with the MP2020 correction scheme should generate output similar to

[CompositionEnergyAdjustment:
   Name: MP2020 anion correction (oxide)
   Value: -5.920 eV
   Uncertainty: 0.014 eV
   Description: Composition-based energy adjustment (-0.740 eV/atom x 8.0 atoms)
   Generated by: MaterialsProject2020Compatibility,
 CompositionEnergyAdjustment:
   Name: MP2020 GGA/GGA+U mixing correction (Fe)
   Value: -8.728 eV
   Uncertainty: 0.036 eV
   Description: Composition-based energy adjustment (-2.182 eV/atom x 4.0 atoms)
   Generated by: MaterialsProject2020Compatibility]

Whereas before, the "Generated by:" line read None

In addition, I discovered that several classes inherited from EnergyAdjustment (including CompositionEnergyAdjustment, which is used by MP2020) were not serializing properly. This was because the description attribute was being modified during init and because some kwargs were not stored as attributes. This PR also fixes those problems.

[mkhorton]

Unless you want to communicate that people should be able to change adj_per_atom after instatiation, you probably want to set it with self._adj_per_atom (similar to eg self._value previously), likewise with uncertainty. Otherwise this looks good.