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.
containers_conversion.hpp: small improvement in readability
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/fix-issue-575-to-matrix
more light updates to doc
Improvement in code of to_matrix
Forgot to remove this (it was included in containers_conversion_test.cpp
Fix cyclic inclusion in containers_conversion
Fix cyclic inclusion in containers_conversion (2)
Fix cyclic inclusion in containers_conversion (3)
Merge pull request #602 from stan-dev/feature/fix-issue-575-to-matrix

to_vector, to_row_vector, to_matrix functions;  ready to go