New version of SIGRES_Plotter (requires abinit 7.5.1)
 New object GSR_File + new approach based on mixin classes to avoid code duplication
 Start to rationalize Abipy Test Suite.
 Use different prefixes for electron and phonon quantities.
 Continue to integrate mixin classes in AbinitNcFile objects.
 Move testing tools to abipy.core.testing
 Rename abipy.tests --> abipy.data