Improve `__str__` and `__repr__`, and implement `_repr_pretty_`

[s-mandra]

At the moment, most of the objects in HybridQ only implement __str__, while __repr__ simply calls __str__. However, __repr__ should return an unambiguous representation of the object while __str__ should return a more readable representation.

Also, it is suggested the implementation of _repr_pretty_ for IPython, to avoid overly verbose output when using Jupyter.