STK_EXPCOV_ISO computes the isotropic exponential covariance function

 CALL: K = stk_expcov_iso (PARAM, X, Y)

   computes the covariance matrix K between the sets of locations X and
   Y, using the isotropic exponential covariance function with parameters
   PARAM. The output matrix K has size NX x NY, where NX is the number of rows
   in X and NY the number of rows in Y. The vector of parameters must have two
   elements:

     * PARAM(1) = log (SIGMA ^ 2), where SIGMA is the standard deviation,

     * PARAM(2) = - log (RHO), where RHO is the range parameter.

 CALL: dK = stk_expcov_iso (PARAM, X, Y, DIFF)

   computes the derivative of the covariance matrix with respect to
   PARAM(DIFF) if DIFF~= -1, or the covariance matrix itself if DIFF is
   equal to -1 (in which case this is equivalent to stk_materncov_iso
   (PARAM, X, Y)).

 CALL: K = stk_expcov_iso (PARAM, X, Y, DIFF, PAIRWISE)

   computes the covariance vector (or a derivative of it if DIFF > 0)
   between the sets of locations X and Y.  The output K is a vector of
   length N, where N is the common number of rows of X and Y.