#458 CIF/BDD bit vectors: rename non-negative classes, move classes to sub-package, introduce common base classes

• Next step in #458.
• Best to review per commit.
• No end-user visible changes.
• Changes:
  • Renamed CifBddBitVector* to NonNegativeCifBddBitVector* and then to UnsignedCifBddBitVector*. Also adapted class JavaDocs where needed.
  • Moved bit vector classes and tests to new bitvectors sub-package.
  • Introduced common CifBddBitVector base class. It has the common fields and methods of all bit vectors. Moved them bit by bit over many commits.
  • Introduced common CifBddBitVectorAndCarry base class. It has the common fields and methods of all bit vector and carry classes. Moved them in one go.
  • Fixed TwosComplementCifBddBitVector constructor JavaDoc.
  • For UnsignedCifBddBitVector, the getInt and getLong methods now return Integer and Long values rather than int and long, with null being returned instead of -1 if the value is non-constant.
  • UnsignedCifBddBitVector now also implements abs.
  • UnsignedCifBddBitVector doesn't support negate.
  • Fixed UnsignedCifBddBitVectorTest.testResizeWithInvalidLength test: it failed on create rather than resize (it effectively tested the wrong method).
  • Added TwosComplementCifBddBitVector.createFromNonNegativeBitVector.
  • Renamed CifBddBitVector.setBits to CifBddBitVector.setBitsToValue.
  • Improved CifBddBitVector.replaceBy JavaDoc.
  • Improve bit vector 'createFromDomain'/'setDomain' methods: improved JavaDocs, improved consistency between the methods, and preconditions are now checked.
  • TwosComplementCifBddBitVector: improved createFrom* methods: improved JavaDoc/comments, improved parameter/variable names.
  • Extended the testCreateFromUnsignedBitVector test.

Addresses #458