simplify the CveCore and CveRecord classes

[hkong-mitre]

There are a total of 4 classes dealing with various depth of CveRecord: CveCore, CveCorePlus, CveRecordV5, and CveRecord.

For CveCore and CveCorePlus, this can probably be merged into a single class now that the delta format is simplified.

For CveRecordV5 and CveRecord, we need to determine how much of the auto generated classes we're actually leveraging, and whether keeping it makes sense anymore.

[hkong-mitre]

address the postponed comment from PR #13 (comment)

[hkong-mitre]

Issue #5 may also be considered by this refactoring.