Created IPathway interface and changed users of Pathway to use IPathway instead

[richard-fine]

Previously, all pathway code was based on the abstract class "Pathway." Because it is an abstract class, rather than an interface, it made it difficult for objects that already derive from another class (e.g. MonoBehaviour) to provide pathway information.

Switching to an interface allows those classes to implement the required methods without losing their existing base class relationship; and because the Pathway class itself supports the interface, no existing code should be affected.

[ricardojmendez]

Hello Richard, thanks for the pull request. I've merged it into master and will integrate it into the development branch as well.