(Not so) Minor sampler refactoring of the samplers

This is the last of my series of MRs and builds on !199 (closed) and !200 (closed) and does a few things to simplify the samplers:

• I got rid of the redundant external_sampler and external_sampler_function attribute
• All samplers now have a static dictionary with default kwargs for the sampler
• These dicts contain all possible kwargs
• The previously obsolete _verify_kwargs_against_external_sampler_function now is called _verify_kwargs_against_default_kwargs and checks against our dict of kwargs
• The kwargs property/setter now has a common template. Child classes only have to implement _translate_kwargs, which handles incorrect kwargs. Most kwargs property/setters could therefore be removed
• Introduced helper classes MCSampler and NestedSampler`
• The methods to setup_nburn and calculate_autocorrelation have been moved to MCSampler, since they don't really make sense outside of this right now
• Some child classes implement an __init__ method now and have some custom attributes
• Dynesty now handles the nested sampler (sampler -> self.sampler) as an attribute internally. This removes the sampler argument in some of the internal methods
• Emcee now has some properties that refer to the respective kwargs
• In case that the external sampler constructs a sampler object and then calls a sampler method, there are now sampler_function_kwargs and sampler_init_kwargs properties, that properly separate those.