AudioSample default argument values cause error when __repr__ called.

[louisfh]

AudioSample does not require arguments for init method, and will use default values of start_time = None and duration=None. However these will raise errors when AudioSample.__repr__ is called, for example in an ipython environment. This is because it tries to calculate the "end time" for printing by calculating start_time + duration.

Raising

TypeError: unsupported operand type(s) for +: 'NoneType' and 'NoneType'

opensoundscape/opensoundscape/sample.py

Lines 31 to 33 in 03e8661

	start_time=None,
	duration=None,
	labels=None,