points_sd: Plot a point with x- and y-error bars
In functClust: Functional Clustering of Redundant Components of a System

Take two vectors, compute their mean and standard deviation, then plot the mean point with error bars along x- and y-axis.

1	points_sd(x, y, opt = c("mean", "mean"), fig, col, bg, cex = 2, name = "")

`x, y`	two numeric vectors, x in abscissa and y in coordinate.
`opt`	two strings, equal to `"mean"` or `"median"`. Indicate if the point should be plotted at `mean(x), mean(y)`, `mean(x), median(y)`, `median(x), mean(y)` or `median(x), median(y)`.
`fig, col`	two integers, indicating the symbol and the colour to use.
`bg`	a string, indicating the colour background to use.
`cex`	a numeric, indicating the size of symbol to plot.
`name`	a string, indicating the label to add near the point.