Ability to configure alternate link-weight function as a default

As a deployer or a developer, I would like to enable different link-weight function as a default for the use by the PathService, so that I do not have to write my own PathService implementation.

Implementation Notes:

• Make PathManager configurable to allow selection from different default behaviours, e.g. hopCount, linkMetric, geoDistance
• Add publicly accessible EdgeWeight function implementations for the above
• Add PathAdminService that allows 'setDefaultEdgeWeight' to allow developers to inject their own custom edge-weight function to use as a default.
• Enahnce BasicLinkConfig to allow linkMetric to be configured externally

Edge Weigths Functions:

• hopCount - +1 for each link
• linkMetric - get linkMetric annotation from each link if present, +1 default
• geoDistance - compute weight as a geo distance using vertex latitude/longitude annotations