The conversion to a NetCDF file should not be automatic

[FrancescAlted]

After discussion with @jfburkhart, we agreed that in case a FLEXPART output is not in NetCDF format, it should be converted automatically. In particular:

fprun = rf.fprun(path, gen_ncfile=False) 

will raise an error in case the FLEXPART output is not in NetCDF format. If the user pass gen_ncfile=True, then the conversion will be done automatically and the result dropped in the same directory than the original output.

[jdavid]

I think now it's Flexpart:

fprun = rf.Flexpart(path, gen_ncfile=False)