Let `Model::froot` depend on `w`

[dweindl]

Currently we have froot(root, t, x, p, k, h, tcl), and w expressions are substituted into the root functions. For some models, this is very costly and results in unwieldy expressions. Let's pass w to `froot'.

[dweindl]

and w expressions are substituted into the root functions

Actually, no: w is only substituted into xdot for computing drootdt_total. Explicit w-dependence of event triggers currently results in compilation errors (can be triggered with the test case in #3031).