(Not so) Minor sampler refactoring of the samplers

CHANGELOG.md

sampler_requirements.txt

test/sampler_test.py

tupak/core/sampler/__init__.py

tupak/core/sampler/base_sampler.py

tupak/core/sampler/cpnest.py

tupak/core/sampler/dynesty.py

tupak/core/sampler/emcee.py

tupak/core/sampler/nestle.py

tupak/core/sampler/ptemcee.py

tupak/core/sampler/pymc3.py

tupak/core/sampler/pymultinest.py