Permutation and naming fix

[ammcpherson]

This pull request addresses issues found during further testing of using SPECFEM3D_Cartesian-generated Green's Functions for force inversions. 2 changes have been made:

1. mtuq/io/clients/SPECFEM3D_SAC.py has been changed to reflect the new naming convention, which is in line with MTUQ's internal coordinate system now. This is also a change for clarity - the new naming convention reminds new users to run 3 simulations for an Up force, a South force, and an East force.
2. mtuq/greens_tensor/SPECFEM3D.py is now also in line with MTUQ's internal coordinate system. This permutation has passed 5 synthetic tests, and one test using the 2009 Anchorage earthquake data. The comparison between a force inversion using the AxiSEM Green's functions (left) and the SPECFEM3D Green's functions (right) is shown below.