Include validation of local files

[bvdh]

I've added support for validation of local files. For this I've updated IGloader to return the loaded resource type and id. This ValidationEngine will use that information to create a R5EvaluationContext class which is passed to the InstanceValidator. This class holds the files as Elements. When a resource is retrieved using FHIRpath, this element is returned and validated.

To test this I've added a test that loads files and validates them.

Is this an approach that would work?