Slow light minor bug and example parameters file

[alejandroc137]

I think the variable tf in at least the models iharm and KHARMA, i.e.,
kharma(ihamr)/model.c
....
// Other parameters
static int reverse_field = 0;
double tf;
....

Is clashing with the one defined in main.c, i.e., this one

static double tf = 0.;

So, I think that writing

extern double tf;

in main.c fixes the problem. I think this is all we did with @gnwong, and it fixed the problem (right, @gnwong?).

Also, it would be great to include an example script to run ipole in the slow light node.

Thank you!