validate
method that has to be implemented to describe the validation-rules
Return
a List of messages (your result-type implementing ValidationMessage)
Parameters
data
model-instance to be validated
metadata
some data to be used as parameters for the validation (validate differently for the steps in a process)
validates the given data by using the given metadata and returns a List of ValidationMessages.
Return
Parameters
data
data to validate
metadata
extra information for the validation process