Get spline coefficients for a single person
individual_spline(person, idvar, yvar, tvar, data, boundaryKnots,
innerKnots, degree, intercept)ID of this person
name of the id variable (string)
the name of the response variable
name of time variable (string)
full dataset
the boundary knots for the bspline
the inner knots for the bspline
the degree of the bspline
whether or not to include an intercept