Fix grid plots on newer matplotlib versions

[b-grimaud]

Grid plots fail on matplotlib 3.9.2 with the following trace :

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[17], line 1
----> 1 mr.plot_waveforms(recgen, electrode='max', cmap='rainbow')

File ~/Documents/GitHub/MEArec/src/MEArec/tools.py:3339, in plot_waveforms(recgen, spiketrain_id, ax, color, cmap, electrode, max_waveforms, ncols, cut_out)
   3336     nrows = 1
   3337     ncols = len(spiketrain_id)
-> 3339 gs = gridspec.GridSpecFromSubplotSpec(nrows, ncols, subplot_spec=ax)
   3341 # find ylim
   3342 min_wf = 0

File ~/.conda/envs/simul/lib/python3.10/site-packages/matplotlib/gridspec.py:490, in GridSpecFromSubplotSpec.__init__(self, nrows, ncols, subplot_spec, wspace, hspace, height_ratios, width_ratios)
    488     self._subplot_spec = subplot_spec
    489 else:
--> 490     raise TypeError(
    491                     "subplot_spec must be type SubplotSpec, "
    492                     "usually from GridSpec, or axes.get_subplotspec.")
    493 self.figure = self._subplot_spec.get_gridspec().figure
    494 super().__init__(nrows, ncols,
    495                  width_ratios=width_ratios,
    496                  height_ratios=height_ratios)

TypeError: subplot_spec must be type SubplotSpec, usually from GridSpec, or axes.get_subplotspec.

The matplotlib blame traces back to this commit which seems to have shipped with the latest version.

The fix involves explicitely calling ax.get_subplotspec() instead of just passing ax.
It doesn't seem to be a breaking change, and I've successfully tested it with matplotlib 3.8.0