Remove references to internal paths

[glatterf42]

message-ix-models/message_ix_models/model/water/data/pre_processing/hydro_agg_basin.py

Line 31 in cf687da

# This is an internal IIASA directory path

Looking more closely at the nexus code, I just noticed that it includes several "internal IIASA paths", which doesn't seem right. No user outside IIASA could run this code, despite it claiming to be open source. So in order to keep this promise, please replace these references with references to commonly accessible locations. You could store the data with the code here (if it's small, <<10MB per file) or store it on zenodo.