get_by_label crash if not str

[sygout]

the function get_by_label does not check the type of the label provided. it then crashes if a float is provided

this case appeared because when reading from an Excel sheet the list of prefLabel, an empty cell is read as a float (NaN).

[francescalb]

Do you think we should check if label is str and return a TypeError if not?

[sygout]

Yes it could be a first step.

Later I think that we should test if the submitted string is a valid label if there are rules for that.

[francescalb]

That we already do. If you ask for a label to does not exist it return NoSuchLabelError.

[francescalb]

closed with #312