Add schema check

This MR introduces an automatic consistency check on the schema of the conditions database.

The check makes sure that all the condition payloads in a .yml folder are consistent (same number and name of top level keys) and enforces that the file .schema.json is in sync with the content of the files.

We are not making any assumption on the internal format of the condition objects as those changes can be easily handled at runtime, while schema changes may lead to failures at the framework (DD4hep) level.

I also fixed a couple of issues in the repository.