Rework GW and BSE method

There are several changes affecting GW and BSE.

1. Added ExcitedStateMethodology in Method as base class for GW, BSE.
2. Added Screening as step to be callable as sub-section in GW and BSE.
3. Fixed some issues with appending and m_add_sub_section in workflow.py.
4. Added a condition for adding inputs in TaskReference normalizer. This was causing that inputs and outputs were repeated.

With this I will move into the normalizer for BSE and Spectra as explained in #1316 (closed).

@ladinesa do you mind reviewing this? It is related with the changes in the parsers. Thanks!