Move SoftwarePackage class to the main SDK

[JAG-UK]

We have the 'SoftwarePackage' class which provides neat entry points and an encapsulation of the NIST minimal Software Bill of Materials as an RKVST Asset. We should move that to the main SDK so that customers can use it directly in their own code, rather than having to dig it out of the samples wheel.

To be clear, the sofwtare_bill_of_materials sample (main.py / run.py) should stay here - it's full of example data and hooky XML files - but it should be modified to import the SoftwarePackage class from its new home in the Jitsuin-archivist SDK.

[eccles]

This issue should also be on the archivist-python repo?