Small refactorings in hazardlib

[micheles]

There is some uglyness in

• openquake/hazardlib/gsim/aristeidou_2024.py, in the functions get_period_im and extract_im_names
• GroundShakingIntensityModel.__init__ contains checks on the class that should go in MetaGSIM.__call__
• most calls to super().__init__() can be removed in the GSIM subclasses
• the function _get_tref is badly written