STK_DATAFRAME constructs a dataframe object

 CALL: D = stk_dataframe (X)

    constructs a dataframe object from X. If X is a plain numeric array, then
    D is dataframe without row or column names. If X already is an
    stk_dataframe object, row names and column names are preserved when they
    exist.

 CALL: D = stk_dataframe (X, COLNAMES)

    allows to specify column names for the dataframe D. Row names from X are
    preserved when they exist.

    If COLNAMES is empty ([]), this is equivalent to D = stk_dataframe (X);
    in particular, if X has column names, then D inherits from them.

    If COLNAMES is an empty cell({}), the resulting dataframe has no column
    names.

 CALL: D = stk_dataframe (X, COLNAMES, ROWNAMES)

    allows to specify row names as well.

    If ROWNAMES is empty ([]), this is equivalent to D = stk_dataframe (X,
    COLNAMES); in particular, if X has row names, then D inherits from them.

    If ROWNAMES is an empty cell({}), the resulting dataframe has no row names.

 NOTE: How to extract the underlying array

    It is sometimes useful to extract from an stk_dataframe object the
    underlying numerical (double precision) array.  This can be achieved either
    using the 'data' field of the object:

       x1 = x.data;        % x1 is a numerical array

    or using a cast to double:

       x2 = double (x);    % x2 is a numerical array too, identical to x1

    Note that the second syntax remains valid even if x is a numerical array
    (single, double, uint8...) instead of an stk_dataframe object.

 See also: stk_factorialdesign, stk_hrect