(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.

added 2 commits

• c17953e8 - Removed output from cpnest default kwargs
• 5e8d8d02 - Uses self._verify_kwargs_against_default_kwargs now to handle all of the stuff…

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added Refactoring label

added 1 commit

• 9808aa99 - Flake8 stuff

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added 2 commits

• 82eddca1 - Fixed some stuff related to removal of self.external_sampler
• 287b8c2e - Reverted back to dynesty sampler

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added 1 commit

• 0f781081 - Fixed a test with nestle

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added 3 commits

• c8b454ec - Added file with sampler requirements
• f2304fb3 - Added sampler requirements to gitlab CI
• fd09e662 - Flake8

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added 1 commit

• 666decb0 - Reverted back to the old state

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added 1 commit

• b17a948b - Removed sampler requirements from the CI

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resolved all discussions

added 10 commits

• d9ba2c55 - A bug
• a32613e7 - Fixed ptemcee
• 74facfb8 - Added default_kwargs and rewrote some stuff to keep everything working
• a3de6280 - Removed redundant import
• 51655243 - Merge branch 'refactor_external_sampler_handling' into minor_sampler_refactoring
• 6acc7cba - changed nwalkers_equiv_kwargs
• 46e42150 - Merge branch 'introduce_translate_kwargs' into minor_sampler_refactoring
• cbfba5d1 - Made the number of walkers/livepoints etc consistent to be 500
• 12f98402 - Added tests for pymc3 kwargs
• 3ca00bce - Introduced NestedSampler/MCSampler helper classes

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• d975a2f1 - Some Flake8 stuff

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added 2 commits

• d090db1c - A docstring
• 0ac5f10c - Reverted back to origin/master

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resolved all discussions

added 6 commits

• 0ac5f10c...205dadd4 - 4 commits from branch master
• 5de3b266 - Merge remote-tracking branch 'origin/master' into minor_sampler_refactoring
• 4107514a - Added some changelogs

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mentioned in issue #185 (closed)

added 1 commit

• a009c346 - renamed _run_external_sampler -> run_sampler

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added 7 commits

• 679dafca - Removed reference to external_sampler_function
• de4e5b7c - Fixed some stuff related to dynesty
• d3f01df0 - Calls NestedSampler init now
• 1d7c44b0 - Fixed some stuff related to emcee
• 2aa89376 - sorted attributes and kwargs properly
• 925f0037 - fixed something in _run_test
• 4bb2f7eb - Fixed a few smaller issues

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added 1 commit

• 78f55354 - Some minor fixes

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added 1 commit

• c99f4465 - Flake8 stuff

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