Oncotree representation now in TopBraid Repository

Instead of a table-based representation of the oncotree, an external TopBraid system provides a proper graph database storage engine and API for representing the OncoTree nodes nodes, relationships, and meta data (such as color, and mainType).

OncoTree versioning is enabled through a separate TopBraid Taxonomy storing all available versions with descriptions and needed identifiers for API and repository access.

Unit testing introduced, focussing on the services / utilities needed for the API.

Some small improvements to the frontend, such as a dropdown selector showing a list of available versions.

Also, partial code cleanup of unused features and api output fields. Updates to spring library components (newer versions).