chore: change fixed L2 names in gui to L2/3

Note that this does not change the L2 names inherent in the `cell_types`
which are loaded dynamically in the gui, such as shown here:
#904 (comment)

hnn_core/gui/gui.py

@@ -400,7 +400,7 @@ def __init__(self, theme_color="#802989",
             button_color=self.layout['theme_color'])
 
         self.cell_type_radio_buttons = RadioButtons(
-            options=['L2 Pyramidal', 'L5 Pyramidal'],
+            options=['L2/3 Pyramidal', 'L5 Pyramidal'],
             description='Cell type:')
 
         self.cell_layer_radio_buttons = RadioButtons(
@@ -1974,6 +1974,9 @@ def _update_cell_params_vbox(cell_type_out, cell_parameters_list,
     if cell_layer in ['Biophysics', 'Geometry']:
         cell_parameters_key += f" {cell_type.split(' ')[0]}"
 
+    # Needed for the button to display L2/3, but the underlying data to use L2
+    cell_parameters_key = cell_parameters_key.replace("L2/3", "L2")
+
     if cell_parameters_key in cell_parameters_list:
         cell_type_out.clear_output()
         with cell_type_out:

hnn_core/viz.py

@@ -327,7 +327,9 @@ def plot_dipole(dpl, tmin=None, tmax=None, ax=None, layer='agg', decim=None,
                 r'$\times$ {:.0f})'.format(scale_applied)
         ax.set_ylabel(ylabel, multialignment='center')
         if layer == 'agg':
-            title_str = 'Aggregate (L2 + L5)'
+            title_str = 'Aggregate (L2/3 + L5)'
+        elif layer == 'L2':
+            title_str = 'L2/3'
         else:
             title_str = layer
         ax.set_title(title_str)