Validation tools

[ajgilbert]

Need a class with a configurable set of checks to perform on a CH instance. These checks would include:

• Checking bin and process names are sufficiently unique. I.e. one observation per bin name, >= 1 uniquely-named processes per bin name
• Ensuring shapes are present for all Observations, Processes and shape Systematics. Verify that the binning is identical in all bins
• Negative histogram bins. Provide options for correcting these - set bins to zero, set to a small positive value, merge with adjacent bins. (though automated bin merging is a big task in its own right)
• Processes with no systematic uncertainties
• Systematic uncertainties not connected to any process
• Bins with no background expectation (and in particular no background but finite signal)
• Maybe give information about pdf parameters that will or won't be getting a constraint
• Could also tie-in the nuisance summary code here, though it's not strictly validation

Will add more things as they come up.