to_vector, to_row_vector, to_matrix functions

#### Summary:

Implement features of issue #575: functions to convert from one container to another: to_vector, to_row_vector, to_matrix, to_array_1d, to_array_2d

#### Intended Effect:

Help with vectorization, etc.

#### How to Verify:

Test were added trying to cover all aspects.

#### Side Effects:

None that I know. Backward-compatibility with the already existent to_vector is preserved.

#### Documentation:

I judged that it was better to create a new chapter for mixed operations.

#### Reviewer Suggestions:

Bob Carpenter