to_matrix and to_vector to apply to general containers

[mike-lawrence]

[edit: generalized for vectors by @bob-carpenter]

Add this signature for to_matrix:

array[N_cols] vector[N_rows] arr_Vec ;
matrix[N_rows,N_cols] mat_from_arr_Vec = to_matrix(arr_Vec) ;//proposed handling of new signature

and general signatures for to_vector for any container:

vector[N] to_vector(some container);

The reason to do this is to make it easy to reshape efficiently for likelihood and prior density increments.

This already works:

array[N_rows] row_vector[N_cols] arr_Rvec ;
matrix[N_rows,N_cols] mat_from_arr_Rvec = to_matrix(arr_Rvec) ;

I've already started a branch of stanc3 exposing this method, but presumably it has to exist in the first place.

[mike-lawrence]

Checking in on this, having just now encountered another scenario not handled by the existing signatures (conversion of a 1D array of simplexes to a matrix, where a simplex is a vector behind the scenes and thus not handled by the current array-of-row-vectors-only signature).