Make Batch client work with PIDClientCredentials

[TobiasWeigel]

The PIDClientCredentials is a good class to conveniently cover all possible authentication cases and get the information either through function calls or from a JSON file.
The batch client has however its own methods. To use both clients transparently, the batch client should be able to work with PIDClientCredential instances as well (e.g. via a generic authenticate() method that takes a PIDClientCredentials instance as only parameter).

[TobiasWeigel]

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.

[SofianeB]

#17 resolves this.

[SofianeB]

First issue solved by #17 and create new issue for #13 (comment)