Internal. Convert point-data duplets (2 variables, x-y
coordinaes) in Clay silta and sand coordinates.
TT.xy2css(xy.data, geo, css.names = NULL, text.tol = NULL,
tri.sum.tst = NULL, tri.pos.tst = NULL, set.par = FALSE,
blr.clock = NULL, text.sum = NULL)
a data.frame with xpos and ypos columns