Bug in HistoryHDF5

[glyg]

I think there's a problem with those lines:

tyssue/tyssue/core/history.py

Lines 287 to 292 in e248cea

	if sheet is None:
	last = self.time_stamps[-1]
	with pd.HDFStore(self.hf5file, "r") as file:
	keys = file.keys()
	if "\cell" in keys:
	sheet = Epithelium

Looks like the code was not completed

TODO:

• write a test
• finish implementation