Allow function call in rule-type translators

[aseeland]

At the moment a rule-translator is defined by trigger (a source_key which value is used to apply the different rules) and individual key-value pairs for each trigger-value with a list of translators.

Another optional key should be added, e.g. trigger_callback, where a string of an implemented function can be given. As a first example, a function should be implemented, that returns the (dataverse-)type of the value as string.
The keys of the individual key-value pairs are then the return values of the trigger_callback (instead of the trigger values).

As a test example the mapping of parameter values to different EngMeta fields (depending on the type, float vs text) can be used.

• add function reference mechanism in yml-configs
• implement get_type as first function that can be referenced in yml-config
• update documentation (service.yaml and README.md)
• add unit test