Add some useful functionalities for new settings handling in dev

[pat-schmitt]

Some useful functionalities to make the workflow with the new settings handling more user-friendly:

• new method for looking at settings gdir.get_setting(var, filesuffix=None): var should be the parameter you are interested in, maybe if var=None return all parameters of the selected settings; filesuffix is the filesuffix of the settings file you are interested in
• new method for changing settings gdir.set_setting(var, value, filesuffix=None, overwrite=False)
• new entitiy task for setting multiple parameters of a single gdir (e.g. tasks.set_setting(gdir, {'var1': value1, 'var2': value2}, filesuffix=None, overwrite=False) ). This also can be used to set parameters for multiple gdirs to the same value with workflow.execute_entity_task
• new method for creating a new settings file with a parent_filesuffix for multiple gdirs (maybe the easiest is again as an entity task and workflow.execute_entity_task)