Reconsider Structure and Vacant

[0ncorhynchus]

The name Structure is confusing because its implementation differs from its name.

The major difference between MoleculePool and StructureType is whether to store coordinates or not.
In addition to this, StructureType have constant values: D = 0.0 and pid = ParticleID(0, 0).
This causes that the particles can't have own ParticleIDs in the case some may want to use StructureType only to ignore particles for tracking.

This is the reason that I propose to rename StructureType with another name (e.g. NoTrackingType) and reimplement it.

Finally, Vacant is also confusing and its attributes are meaningless.
So, we can replace it with null_ptr (because we use them as shared_ptr<VoxelPool> or raw pointers).

related to #334.