Photometry handler: pandas warning

[Theodlz]

#2648 added a line (L479) where if the origin of a photometry point is None, it is set to the string "None". I'm not sure why this is needed, but now it raises a pandas deprecation warning. In future releases, having None in a column will raise an error because the dtype of the column is float64 and so it doesnt know what to cast that string too.

We should make sure that the dtype of the origin column is string.