Fix `__repr__` for DeltaFunction

The DeltaFunction prior has __repr__

>>> a = tupak.prior.DeltaFunction(1, name='test')
>>> print(a)
Out: DeltaFunction(name='test', latex_label='test', unit=None, minimum=1, maximum=1, peak=1)

But, trying to instantiate a DeltaFunction instance with a minimum and maximum argument will return

TypeError: __init__() got an unexpected keyword argument 'maximum'

Either, the __repr__ should be fixed, or it should accept a minimum and maximum value.