tf: approximate trend filtering via MM algorithm
In btf: Estimates Univariate Function via Bayesian Trend Filtering

Uses majorization-minimization technique to approximate trend filtering fit.

1 2	tf(y, x = NULL, k = 2, l, D = NULL, eps = 1e-08, tau = 1e-05, max_iter = 500)

`y`	observed data
`x`	inputs corresponding to observations
`k`	order of fit
`l`	vector of penalty parameters lambda
`D`	matrix Delta of order k+1
`eps`	error adjustment to majorization function
`tau`	convergence threshold
`max_iter`	maximum number of iterations allowed