Missing check for references to unknown feeder vehicle journey in an interchange

[vpaturet]

The rule in chouette 3-Interchange-2 checks for references to unknown feeder vehicle journey in an interchange.
It seems that this has been only partially implemented in antu:

antu/src/main/java/no/entur/antu/validation/validator/interchange/mandatoryfields/MandatoryFieldsValidator.java

Line 91 in 76d701d

// 3-Interchange-2

This report shows the error caught by chouette:
https://rutedb.entur.org/referentials/mor/compliance_checks/8237163/report