Implement parametrized histogram as a class

[ajgilbert]

Would be constructed from the json format. Should support:

• creating a TH1 for a given value of the params
• multiplying the scaling terms but another scaling term
• writing back to the json format

[ajgilbert]

Mostly fixed by #26. Some extra work needed for the scaling by multiple EFT parameterisations (i.e. prod x partial_width / total_width) for Higgs. Should also document usage of the new classes.