getf: Get RGAM model component for one feature
In relgam: Reluctant Generalized Additive Models

Returns the additive component of the RGAM model for a given feature at given data points, i.e. f_j(X_j).

1	getf(object, x, j, index)

`object`	Fitted `rgam` object.
`x`	Data for which we want the additive component. If `x` is a matrix, it assumed that `X_j` is the `j`th column of this matrix. If `x` is a vector, it is assumed to be `X_j` itself.
`j`	The index of the original feature whose additive component we want.
`index`	Index of lambda value for which plotting is desired. Default is the last lambda value in `object$lambda`.

set.seed(1)
n <- 100; p <- 20
x <- matrix(rnorm(n * p), n, p)
beta <- matrix(c(rep(2, 5), rep(0, 15)), ncol = 1)
y <- x %*% beta + rnorm(n)

fit <- rgam(x, y)

# get the additive component for the feature 6, x as matrix
f6 <- getf(fit, x, 6)  # last value of lambda
plot(x[, 6], f6)
f6 <- getf(fit, x, 6, index = 20)  # f1 at 20th value of lambda
plot(x[, 6], f6)

# get the additive component for the feature 6, x as vector
new_x6 <- seq(-1, 1, length.out = 30)
new_f6 <- getf(fit, new_x6, 6)  # last value of lambda
plot(new_x6, new_f6)