STK_ISNOISY returns false for noiseless models, true otherwise

 CALL: ISNOISY = stk_isnoisy (MODEL)

    returns false for a noiseless MODEL and true otherwise.

    MODEL is considered noiseless if:

     * MODEL.lognoisevariance is missing or empty  (for backward compatibility
       with previous versions of STK where the lognoisevariance field was
       optional),

     * MODEL.lognoisevariance is equal to -inf  (this is the default for a
       model created by stk_model and the currently recommended way to declare
       a noiseless model),

     * MODEL.lognoisevariance is a vector of -inf  (heteroscedastic case with
       all variances set to zero).

 REMARK: Variance model objects

    Note that in the case where MODEL.lognoisevariance is an object, the MODEL
    is automatically considered noisy, even when the parameters of the variance
    model are set (or can be set) to values such that the noise variance
    function vanishes.

 See also: stk_model