Fix git dependency for product generation

[albangossard]

For the moment to generate products we rely on the git hash. This means that the product has to be generated inside a MBM repository. @JoachimPiret spotted an issue that we should fix when running:

import massbalancemachine as mbm
import sys
sys.path.append('/home/joachim/MassBalanceMachine')
rgi_region = 11
cfg = mbm.Config(metaData = ["RGIId", "ID", "N_MONTHS", "MONTHS"])
data = mbm.data_processing.wgms.load_processed_wgms(rgi_region=rgi_region)
mbm.data_preprocessing.wgms.build_monthly_data(data, cfg, rgi_region)

InvalidGitRepositoryError: /home/joachim/MBM_data/notebook/wgms

Not clear what would be the best strategy. For sky view factors we don't really need the git hash so I think we may drop this dependency.