deprecate argument 'constraint' in bbs()

[sbrockhaus]

As @hofnerb pointed out that bbs() with argument constraint should not be used for decreasing or increasing effects, this argument should be deprecated.
Instead bmono() can be used.

Consider an example where the true function is not monotonic:

library(mboost)

x <- sort(rnorm(100))
y <- x^2
y <- y - mean(y)
dat <- data.frame(y = y, x = x)

m <- mboost(y ~ bbs(x), data = dat)
m_bbs <- mboost(y ~ bbs(x, constraint = "increasing"), data = dat)
m_bmono <- mboost(y ~ bmono(x, constraint = "increasing"), data = dat)

par(mfrow = c(1,3))
plot(m, main = "bbs(x)"); lines(x, y, col=2)
plot(m_bbs, main = "bbs(x, constraint = \"increasing\")"); lines(x, y, col=2)
plot(m_bmono, main = "bmono(x, constraint = \"increasing\")"); lines(x, y, col=2)

@Almond-S

[hofnerb]

We now discourage using bbs(, constraint != "none"); However, we cannot remove this functionality as other code and other packages (ctm) rely on it.