Define credible interval object

In !446 (merged) @sylvia.biscoveanu added a confidence_interval method. This is super useful and I can see lots of people making use of it. Before it becomes widely used (and hence we can't monkey around with it), it would be good to generalise it.

Ideally, we should have a CredibelInterval object which behaves as such

>>> ci = CredibleInterval(data, level)
>>> print(ci.lower_absolute, ci.upper_absolute, ci.median, ci.lower_relative, ci.upper_relative)

such that ci.lower_absolute is the value of the lower C.I while ci_lower_relative would be the relative offset (not sure that naming convention is the best, but you get the idea hopefully).

It could also do neat things like

>>> str(ci)
"10.2^{+0.2}_{-0.3}"