frml.additive.smooth: Formula for GAM without crossterms
Description
Creates a formula for gam to be used in regrXonS. For data $X=(X_1,...X_n,X_{n+1},...,X_m)$, variable to be regressed $X_{i}$, i=1...n and variables to regress on $S={X_{n+1},...,X_m}$ creates formula $X_i ~ s(X_{n+1}) + ... + s(X_m)$.