Improvements on non-atomic value validation

The record validation for nested structures could be standardized. Currently, it seems like the validation depends on the DTR validation schema. For example, the checksum validation, a valid value entry would be: "{'md5sum': '723140e4864011bdf1fbc66698a0f041'}". However, if there is no validation schema provided in the DTR for a PID-Info Type, there is no validation for nested structures at all. Furthermore, it would be more reasonable to put the PID of the keys within such structures instead of their name, as only the PID is persistent and recognizable by clients. In the checksum example this would then look like this: "{'21.T11148/ef277087753e8ba2e606': '723140e4864011bdf1fbc66698a0f041'}"

Start by tracing record validation for nested structures and how the DTR validation schema is applied. Done means nested values are validated consistently even without a DTR schema, with PID identifiers used as keys instead of names.

Written by the indexing model from the issue text.