Modified the "extrap" option for interp2, interp3, and interpn (bug #44002). * interp2.m: allow only "spline" method to set extrapolation values, improved input validation, updated documentation, testcases added and modified * interp3.m: allow only "spline" method to set extrapolation values, improved input validation, updated documentation (no need to implement "pchip"), testcases added and modified * interpn.m: allow only "spline" method to set extrapolation values, improved input validation, updated documentation, testcases added and modified * __splinen__.m: only apply external supplied extrapolation value