If pars is a parameter list (of the form returned by sk_pars), the
function returns a list of strings: a kernel family string ('k'), dimension-wise kernel
parameters (sub-list 'kp', with entries 'y' and 'x'), and a title containing the
kernel family, nugget effect, and partial sill.
When pars is a character string, the function returns it unchanged. When pars
is a vector of two character strings, it concatenates them with separator " x ".
When pars is a list of kernel parameters for a single dimension, it returns a named
list of two character strings: the kernel name (named entry 'k') and the parameter(s)
(named entry 'kp'), parenthesized, in "name = value" format where "value" is rounded
to nsig significant digits).