v0.5.4 - Update to Go import paths

This release migrates the Go library import paths to use the familiar Go-gettable github.com prefixed import paths (and moves the code location to /go/jsonformat). This is the first step in making the Go library Go mod compliant. Current users of the library will need to migrate their import paths from google/fhir/jsonformat/jsonformat to github.com/google/fhir/go/jsonformat when upgrading to this new version.

Additional changes include some refreshes to the machine-learning code, including making the C++ ML code version independent, and bumping the Python ML code from STU3 to R4. This is in anticipation of an upcoming major release that will separate TensorFlow/ML code into a standalone library that runs with FhirProto as a dep.