Rename imports from ase.ga to ase_ga

[pa233796]

Starting with ASE version 3.27 the ase.ga module was moved to a standalone project, ase_ga. This causes the following error when attempting to use oganesson with a new installation:

ImportError: Cannot import ase_ga.
The ase.ga code has moved to a separate project, ase_ga:
https://github.com/dtu-energy/ase-ga .
Please install it using e.g. pip install ase-ga.
Please import from ase_ga what would previously be imported from ase.ga.
ase.ga placeholders will be removed in a future release.

PR simply adds ase_ga as dependency and fixes imports. Otherwise, consider locking the ASE dependency to 3.26.