Update __init__.py to check if client is callable

[nilsoberg]

When assigning clients to attributes in the Clients class, check if the value is callable. This allows us to do things like this in the main module Impl file, and pass those objects into the Clients class when creating an object that inherits from Core:

config = dict(
    callback_url=self.callback_url,
    shared_folder=self.shared_folder,
    clients=dict(
        KBaseReport=KBaseReport(self.callback_url, token=self.token),
        DataFileUtil=DataFileUtil(self.callback_url, token=self.token),
        Workspace=Workspace(self.config["workspace-url"], token=self.token)
    ),
)