Check if user is instance admin in openBIS does not work for groups

[wow-such-code]

openbis-client-lib/src/main/java/life/qbic/openbis/openbisclient/OpenBisClient.java

Line 578 in 0c74a37

public boolean isUserAdmin(String userID) {

Expected behaviour:
Usernames that are not themselves assigned the instance admin role, but are part of a group that is assigned as instance admin, should also make the function return "true"

Observed behaviour:
Method returns false for usernames that are part of a group that has "instance admin" rights. the return value is "true" only if usernames themselves are assigned the instance admin role