Create PartitionVector class (#759726)

Just creates PartitionVector class and includes it in partition.h so
that it is built and validated by the compiler.  Not used anywhere yet.

Implementation strategy is to create a PartitionLUKS class derived from
the Partition class.  This implies polymorphism of Partition objects,
which in C++ requires using pointers and references to objects, and not
using objects directly.  (See C++ object slicing).  Later this
PartitionVector class will be modified to use pointers to Partition
objects and act as the owner of the pointed to Partition objects.

Bug 759726 - Implement Partition object polymorphism