Added new persistent sampler that does not change the chain state, useful for forward sampling using generated quanities only
Rename dist -> distance and squared_dist -> squared_distance, enable squared_distance in the modelling language and readd the reference documentation.
Missed %
Fixed header guard issue.
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
Test mdivide_{left|right}_ldlt and log_determinant_ldlt.
Split trace_gen_quad_form tests into their own files.
Add unit tests for trace_inv_quad_form_ldlt and fix derivative error.
Rename trace_inv_quad_form_ldlt with three arguments to trace_gen_inv_quad_form_ldlt to be consistent with the non-inverse form of the trace-of-quadratic-form functions.
Rename test functions to be consistent.
Unit test for trace_gen_inv_quad_form_ldlt.
Expand testing for multi_student_t_log.
Use LDLT_factor in multi_normal_log again now that the gradients are fixed.