Add identity mapping API to the identitylib

Closes #9 (closed)

Also added some clean-up to the project to fix issues with running tests locally. The docker-compose configuration was binding the local files onto the working files in the docker container. For most projects this is appropriate, but in this case the bound volume was shadowing the generated identitylib source files, so in this case I have removed this volume.

This means that the container needs to be rebuilt before running tests, rather than automatically picking up local changes as in most other projects. I think this is an acceptable trade-off to allow the tests to run at all!