STK_LENGTH return the "length" of an array

 CALL:  L = stk_length (X)

    returns the "length" of the data array X.

 NOTE:

    Currently, the length of a data array (numeric array or stk_dataframe
    object) is defined as SIZE (X, 1) but this might change in future
    versions of STK. Note that this is not the same as length (X).