Handle common code between training and inference pipelines

[ekauffma]

Currently, the training and inference pipelines in the CMS open data ttbar notebook have overlapping code which is simply duplicated at this point. This will be difficult to maintain, so it would be good to import these common features from one file.

Commonalities:

• Calculation of top mass using trijet reconstruction method
• Cuts
• Options for processing

[alexander-held]

Some of the visualization code in the training notebook also looks quite similar, perhaps there it is possible to factor out functionality to utils as well and to re-use it for plotting.