Extend metadata model

[severinsimmler]

At the moment Metadata consists of information from the file paths. This is to be expanded, a corresponding class already exists:

cophi-toolbox/src/cophi/model.py

Lines 608 to 614 in 2c8a8f1

	class Metadata(pd.DataFrame):
	"""Handle corpus metadata.
	Feel free to implement some fancy stuff here.
	"""
	def __init__(self, **kwargs):
	super().__init__(**kwargs)