Add support for one-sided uncertainties

[GraemeWatt]

Related to #87 ("Add support for relative uncertainties"), the Uncertainties section of the HEPData submission documentation mentions that a one-sided uncertainty can be represented using an empty string, e.g. asymerror: {plus: '', minus: -0.3}, but this is not yet supported by hepdata_lib.

[AndreasAlbert]

Wouldn't it make more sense to use: {plus: 0, minus: -0.3}?

[GraemeWatt]

Yes, I think mostly giving 0 should be fine. But it was a request from ATLAS a few years ago (see HEPData/hepdata#122) to support one-sided uncertainties as being undefined in one direction rather than strictly zero (see example table). I don't think many records have used this feature, but I thought that since HEPData supports it, then so should hepdata_lib. I wouldn't give it a high priority, but it might be worth implementing at the same time as percentage uncertainties (#87).

[clelange]

Fixed by #169