Added new persistent sampler that does not change the chain state, useful for forward sampling using generated quanities only
Merge branch 'develop' into feature/persistent_sampler

Conflicts:
	make/tests
Fix broken test, update doc
making test clearer
Changed name from persistent sampler to fixed parameter sampler
Merge branch 'develop' into feature/persistent_sampler
Updated tests
Merge branch 'feature/persistent_sampler' of github.com:stan-dev/stan into feature/persistent_sampler

Conflicts:
	src/test/mcmc/fixed_param_sampler_test.cpp
Working empty sampler test
Merge branch 'develop' into feature/persistent_sampler

Conflicts:
	make/tests
Merge pull request #353 from stan-dev/feature/persistent_sampler

Feature/persistent sampler