Investigate PIDClientCredentials class when instantiating a search

[SofianeB]

One further issue may be that if private key authentication is to be used, it is logically nto required to provide a password in the credentials file. However, omitting a password will cause the credentials file read method to throw an error, complaining that without password, searchign will not be possible.
Suggested solution: modify credentials JSON read method so that possibility for searches is not checked if a specific flag parameter is given. Or make a subclass that is specific for the REST mode and only there the error will be thrown.
Text from #13