blood-brain barrier suggested revisions

Right now the blood-brain barrier has no relationship to the vascular system in UBERON:

The suggested revisions are based on a recent blood-brain barrier review from the Zlokovic lab: PMID:30280653:

• Should the ‘neurovascular unit’ be a synonym of the ‘neurovascular bundle’ (UBERON_0016630)?
  [see Figure 1 in a recent review from the Zlokovic lab: https://www.ncbi.nlm.nih.gov/pubmed/30280653];
• Should the ‘blood-brain barrier’ be a part_of child of ‘neurovascular bundle’ (UBERON_0016630)?
  [see Figure 1 in a recent review from the Zlokovic lab: https://www.ncbi.nlm.nih.gov/pubmed/30280653];
• Should the ‘blood brain barrier’ also have a relationship to either the vascular plexus (UBERON_0005629), or vasculature (UBERON_0002049)?
  [see section II. A. ‘BBB formation’ in a recent review from the Zlokovic lab: https://www.ncbi.nlm.nih.gov/pubmed/30280653].

cc @RLovering

Review the definitions and existing relationships for UBERON_0016630, UBERON_0005629, and UBERON_0002049, then compare the proposed modeling to Figure 1 and section II.A of PMID:30280653. Done means reaching a decision on the synonym and vascular relationships and recording the agreed ontology updates.