CT source of dose information: extract to new table and move existing data

[Ed McDonagh]

Source of dose information should be a 1-n field, so has to have its own table. Information could potentially be from a mixture of sources.

Change the extraction code to use the new table and allow for more than one source, and move the existing data to the new table.

Check to ensure that there aren't any display or export views that use the field - if they are, update them.

[Ed McDonagh]

Added new SourceOfCTDoseInformation table to contain potentially more than one source. Used CamelCase for class name, as per PEP8, which is obviously different to the rest of the models. Refs #55. Created ref #164 to implement the change in the extractor code.

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