Add seletop module

[rvhonorato]

For haddock to work in the classical way we also need a model to select the top X models from a previous stage. In the production version the rigid-body generates 1000 models, then the top 200 are selected to the next stage.

This can be done in haddock3 with the seletop module; it gets all the models from the previous_io and ranks them according to their score and outputs a subset equal to select

[joaomcteixeira]

seletop.toml must be renamed to defaults.toml

[joaomcteixeira]

Looks a very clean addition. Can you please change the lines I suggested and that is it, it will read much cleaner. Great job! Good team!