The 'gpr' object from either training or predicting of the Gaussian
Process.
fitted
Logical. Plot fitted values or not. Default to FALSE. If FALSE,
plot the predictions.
col.no
Column number of the input matrix. If the input matrix has more
than one columns, than one of them will be used in the plot. Default to be
the first one.
ylim
Range value for y-axis.
realisation
Integer identifying which realisation should be plotted
(if there are multiple).