Add merge_gridded_data to tutorials

[pat-schmitt]

Add an explanation of merge_gridded_data somewhere in the tutorials, I am not sure at the moment at which place.

Adapt distribute_flowline tutorial:

• Add an explanation of new merging functionality for dynamic runs to this tutorial, added in PR https://github.com/OGGM/oggm/pull/1674.
• Also need to rename distributed_thickness to simulated_thickness in this notebook.

[bearecinos]

Hey @pat-schmitt I found another issue with the tutorials ... in distribute_thickness_from_simulation you assume that add_smoothed_glacier_topo and assign_points_to_band have been run before that task but the tutorials do not reflect that.

 # This is to add a new topography to the file (smoothed differently)
workflow.execute_entity_task(distribute_2d.add_smoothed_glacier_topo, gdirs)
# This is to get the bed map at the start of the simulation
workflow.execute_entity_task(tasks.distribute_thickness_per_altitude, gdirs)
# This is to prepare the glacier directory for the interpolation (needs to be done only once)
workflow.execute_entity_task(distribute_2d.assign_points_to_band, gdirs);

Maybe modify the order above in the notebook. Took me a bit to work it out.