Minor refactor: Make creation of Experiment objects more explicit

[lebrice]

• Arguments are fully specified in Experiment.__init__. This is so we can avoid the current pattern of setting the attributes on the instance after it is initialized.
• Adds type annotations to the arguments of Experiment.__init__ (and the attributes of Experiment objects), with typed dictionaries for the refers and metadata attributes. (Edit: Will be in another PR).
• Add type annotations to some of the functions in experiment_builder.py
• Raise a UserWarning if experiment_builder.create_experiment receives extra unused keyword arguments.