Installation needs `cython` and problem installing `seislib` on MacOS

[jwhhh]

Currently installing Espresso directly in a new environment will cause a ModuleNotFoundError, since installing seislib needs cython. This needs to be resolved either by adding cython into the installation step or (ideally) listing cython as a dependency of seislib.

Another issue is on MacOS, where the following can occur on some machine:

Collecting seislib>=0.6.11
  Using cached seislib-0.6.12.tar.gz (1.7 MB)
  Preparing metadata (setup.py) ... error
  error: subprocess-exited-with-error

  × python setup.py egg_info did not run successfully.
  │ exit code: 1
  ╰─> [8 lines of output]
      Traceback (most recent call last):
        File "<string>", line 2, in <module>
        File "<pip-setuptools-caller>", line 34, in <module>
        File "/private/var/folders/kd/vg4zlbd56znbhb0dvl68rcb40000gp/T/pip-install-t_7nsknw/seislib_8a63ef28e86643f99b2669ebc59bc713/setup.py", line 98, in <module>
          ext_modules=setup_cython_extension(),
        File "/private/var/folders/kd/vg4zlbd56znbhb0dvl68rcb40000gp/T/pip-install-t_7nsknw/seislib_8a63ef28e86643f99b2669ebc59bc713/setup.py", line 59, in setup_cython_extension
          versions = os.listdir('/usr/local/Cellar/gcc/')
      FileNotFoundError: [Errno 2] No such file or directory: '/usr/local/Cellar/gcc/'
      [end of output]

Help needed from @fmagrini