Enforce saving fit data to expected filename

[duetosymmetry]

In base class SurFinBH, a model named with name <name> is expected to be loaded from fit_<name>.h5:

surfinBH/surfinBH/surfinBH.py

Line 72 in fbff946

h5file = h5py.File('%s/fit_%s.h5'%(DataPath(), name), 'r')

But when fetching in DownloadData, the destination filename is just taken from the URL:

surfinBH/surfinBH/_loadFits.py

Lines 59 to 61 in fbff946

	print('Downloading %s data'%name)
	data_url = fits_collection[name].data_url
	filename = data_url.split('/')[-1]

Maybe we should save to fit_<name>.h5 so that data_url isn't restricted.