Document the new tutorial on ingesting velocity data and compare it to the model output

[fmaussion]

The skeleton is here: https://github.com/OGGM/tutorials/blob/master/notebooks/ingest_gridded_data_on_flowlines.ipynb

[bearecinos]

Hey I am happy to do this ! is its still an issue ... We should also add the error part as well... can do

[fmaussion]

Yes the tutorial is definitely empty : https://github.com/OGGM/tutorials/blob/master/notebooks/ingest_gridded_data_on_flowlines.ipynb

;-)

[minallah]

Happy to work on this tutorial - I'll be focusing on the HMA region.