Externalizing update PET json's

[bendhouseart]

Moving "generalize-able" methods out from more specified classes, updating ecat.py with lastest functionality in the process, and ultimately adding on CLI's.

CLI's will enable a user to input a pet json file and a dicom, spreadsheet, or ecat file to update the values within the supplied json.

Closes #255

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📚 Documentation preview 📚: https://pet2bids--262.org.readthedocs.build/en/262/

[bendhouseart]

Mostly there, moved all of the shared json checking and updating functions into update_pet_json.py and added entry points for updating a pet json from either an ECAT or from a dicom to the command line to their respective places. Just need to make sure that check json and other PET based "QC" methods get applied to the newly minted command line versions. I was hoping to get this all into 1 command line function, but I see myself falling into circular imports with how things are currently set up for ECAT. I would really like to avoid that, so compromise for now.

Some day update json will take either dicom or ecat and be contained to a single function.