Feat/str format

[fjosw]

This PR adds a __format__ method to the Obs class and extends the functionality of _format_uncertainty.
For example the following code

o0 = pe.pseudo_Obs(-0.3, 0.421, "test")
o1 = pe.pseudo_Obs(0.21, 0.1452879, "test")
for i, o in enumerate([o0, o1]):
    print(f"Obs{i} {o:+3}")

now outputs three significant digits and adds a + sign to non-negative observables:

Obs0 -0.300(421)
Obs1 +0.210(145)

[s-kuberski]

Hi! Thanks for implementing this, I think it is a nice feature. Did you think about adding some more features (fill with whitespaces, scientific notation, alignment.... as outlined, e.g., here https://python-reference.readthedocs.io/en/latest/docs/functions/format.html) to the routine? It could be helpful for creating text output if some of the features that are supported for floats could be used for obs, as well.

[fjosw]

Thanks for the feedback. I now implemented "+", "-" and " " as in the documentation. For now I will not look into scientific notation or alignment. If needed alignment could be achieved with nested f-strings, for example:

print(f"{f'{o1:+3}':<20}text")

+0.3480(123)        text