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.
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
containers_conversion.hpp: small improvement in readability
refactor: moved existing check_* functions to src/stan/math/error_handling__/
adding unit test for dom_err.
refactoring: reimplemented dom_err to pass tests
adding unit test for dom_err_vec
adding implementation for dom_err_vec
refactoring: error_handling/check_bounded.hpp
refactoring: error_handling/check_consistent_size
refactoring: removed second check_bounded function signature
refactoring: error_handling/check_consistent_sizes.hpp
adding dom_err and dom_err_vec that do not have a result parameter
removing dom_err and dom_err_vec that do not have a result parameter
refactoring: tried to find all check_* references that didn't have a final parameter and filled it in. Should not change behavior
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
refactoring: reimplemented error_handling/matrix
moved errors to math/error_handling/

check_finite, check_greater, check_greater_or_equal, check_less,
check_less_or_equal, check_nonnegative, check_not_nan, check_positive;
and removed validate_non_negative_rep which duplicated check_nonnegative
fix merge conflict