Added to_matrix, to_row_vector, to_array and flatten. Extended to_vector
Changes and improvements to to_matrix, to_row_vector, to_array_1d, to_array_2d, to_vector. flatten renamed to array_1d.
enhance censoring docs

- y_obs should be declared unconstrained
- use array sampling operators
- use normal_ccdf_log
- clarify the type of censoring used
- provide an example of left-censored data
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
containers_conversion.hpp: small improvement in readability
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge pull request #639 from alyst/fix_censoring_docs

I proofread the text and models and made sure the model changes compiled.
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/fix-issue-575-to-matrix
more light updates to doc
added README for Stan Development Team clarification
Improvement in code of to_matrix
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Forgot to remove this (it was included in containers_conversion_test.cpp
Fix cyclic inclusion in containers_conversion
numbers out of range throw exception, added test to check exception to unit tests
Fix cyclic inclusion in containers_conversion (2)