Enable NVD cache search without SBOM

[mdingwall]

Need to add the ability to make the CveMatcher class be given a list of CPEs to search for. This should allow it to find packages without the need for an SBOM document.

Tasks:

• Look at changing the CveMatcher.init() method to no longer take an SPDXDocument at initialization.
• Add properties for an SPDXDocument or CPEList.
• Move the SPDXDocument processing from CveMatcher.init() into a new method that is called by the user.
• Make new usage pattern for CveMatcher:

  ...
  # Rough pseudo code
  matcher = CveMatcher(VulnerabilityDabase.path())
  if args.file:
    sbom_file = SpdxDocument.open(args.file) # this is not workable code, assumes only a single SPDX file
    matcher.spdx_document = sbom_file
  elif len(args.packages) > 0:
    #package_list = process_packages_to_cpes(args.packages)
    #matcher.package_list = package_list
    for package in args.packages:
      matcher.add_package(process_package_arg(package))
  matcher.process()