New labelling system for LineageTree

[BadPrograms]

Labels are now a new class that works like a dict, but tests if all values of the dictionary are actually strings, can be labels. These labels are saved inside a new attribute of LineageTree called labelling.
lT.labelling contains all the labels and tests if all new labels correspond to a subset of the nodes or the set of the nodes.
To access these labels:

lT = ...
lT.labelings.list_of_labels
# ["default_dict", "label1", "label2",...]
lT.labellings["default_dict"][node_id]
# prints the label of the node

get_ancestor_with_attribute works the same way as before.
get_labelled_ancestor works the same way as before.

I am open to discuss if we should drop labelling and save everything directly on LineageTree.