seaice diags needs refactoring

[raphaeldussin]

plot function does not work:

fig = plot(model, obs, regridded, valid_mask)                                                                                                                                                                          
/net2/rnd/anaconda3/envs/repro/lib/python3.8/site-packages/cartopy/mpl/geoaxes.py:1491: MatplotlibDeprecationWarning: shading='flat' when X and Y have the same dimensions as C is deprecated since 3.3.  Either specify the corners of the quadrilaterals with X and Y, or pass shading='auto', 'nearest' or 'gouraud', or set rcParams['pcolor.shading'].  This will become an error two minor releases later.
  X, Y, C = self._pcolorargs('pcolormesh', *args, allmatch=allmatch)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-20-8404f36099d9> in <module>
----> 1 fig = plot(model, obs, regridded, valid_mask)

/net2/rnd/anaconda3/envs/repro/lib/python3.8/site-packages/om4labs-0.0.1-py3.8.egg/om4labs/diags/seaice/seaice.py in plot(model, obs, regridded, valid_mask, label, region, month)
    272     plotdata = (model["ac"][month_index] * 100.0).to_masked_array()
    273     plotdata = np.ma.masked_where(valid_mask, plotdata)
--> 274     cb1 = _plot_map_panel(ax, x, y, plotdata, extent=extent)
    275     ax.set_title(f"Model - Years {model.time[0]} to {model.time[1]}")
    276     fig.colorbar(

/net2/rnd/anaconda3/envs/repro/lib/python3.8/site-packages/om4labs-0.0.1-py3.8.egg/om4labs/diags/seaice/seaice.py in _plot_map_panel(ax, x, y, plotdata, cmap, vmin, vmax, extent, contour)
    235     cmap.set_bad(color="#555555", alpha=1)
    236     ax.set_extent(extent, ccrs.PlateCarree())
--> 237     cb = ax.pcolormesh(
    238         x, y, plotdata, transform=ccrs.PlateCarree(), cmap=cmap, vmin=vmin, vmax=vmax
    239     )

/net2/rnd/anaconda3/envs/repro/lib/python3.8/site-packages/cartopy/mpl/geoaxes.py in pcolormesh(self, *args, **kwargs)
   1457                              ' consider using PlateCarree/RotatedPole.')
   1458         kwargs.setdefault('transform', t)
-> 1459         result = self._pcolormesh_patched(*args, **kwargs)
   1460         self.autoscale_view()
   1461         return result

/net2/rnd/anaconda3/envs/repro/lib/python3.8/site-packages/cartopy/mpl/geoaxes.py in _pcolormesh_patched(self, *args, **kwargs)
   1489         allmatch = (shading == 'gouraud')
   1490 
-> 1491         X, Y, C = self._pcolorargs('pcolormesh', *args, allmatch=allmatch)
   1492         Ny, Nx = X.shape
   1493 

ValueError: too many values to unpack (expected 3)

too many computations/reductions are made in the plot function which is against the design of having calculate output numpy arrays ready for the plot part