STK_OPTIM_ISAVAILABLE checks if a given optimizer is available

 CALL: B = stk_optim_isavailable (ALGO)

    return true is ALGO is available, and false otherwise.  ALGO can be the
    short name of an algorithm (such as 'octavesqp'), the corresponding class
    name (such as 'stk_optim_octavesqp'), a handle on the class constructor
    (such as @stk_optim_octavesqp) or an algorithm object.