Update fit_catalogue.py

[PJ-Watson]

This line throws the following error in my environment with pandas==3.0.3 and numpy==2.4.6 (and this is the only line which has caused any problems with those two package versions):

Traceback (most recent call last):
  File "/media/sharedData/python/py3.13_PIE/code/PASSAGE_scripts/3__SED_fitting/run_fits.py", line 214, in <module>
    cat_fit.fit(n_live=400, verbose=True, mpi_serial=False, track_backlog=True)
    ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/media/sharedData/python/py3.13_PIE/lib/python3.13/site-packages/bagpipes/catalogue/fit_catalogue.py", line 190, in fit
    self.done[i] = True
    ~~~~~~~~~^^^
ValueError: assignment destination is read-only

I'm pretty sure this is the new pandas 3.0 copy-on-write behaviour (or less likely, some other minor update to numpy that I overlooked), so creating self.done explicitly as a numpy array solves the problem.