Add methods for artifact-based property predictors

[jannisborn]

Is your feature request related to a problem? Please describe. Currently the artifact-based property predictors (like gt4sd.properties.molecules.core.Tox21) are not usable as functions via gt4sd.properties.molecules.tox_21, unlike all the non-artifact-based properties). Moving the functions there would yield circula import issues

Describe the solution you'd like A small refactor that goes around the circular imports