Emulator changes

[lauracabayol]

Some new changes to the previous version in the input_emulator branch:

• Add the option of drop_z to the archive. Please @jchavesmontero check the base_archive changes I introduced.
• Add option drop_sim and drop_z to the emulators
• Clean the obtain_params emulator function. Now it does not return Nz, but it was not used and as pointed out by Andreu, it was dangerous for simulations with missing snapshots
• In the same function, create on mask in k per snapshot since there are simulations with different number of k's. Use this mask instead of a hardcoded Nk option that was causing problems when running with Nyx simulations

[lauracabayol]

Also, I forgot that there is a temporary hack in the Nyx_archive to get the sims redshifts used for the leave_one_out test