Default parscale scaling won't work for -1..1

[lentinj]

mean(c(-1, 1)) obviously isn't what we want.

• Do something avoid_zero()-esque
• Do something else, e.g. taking the closest to zero of the 2?

[lentinj]

diff() is the obvious answer here. But the setting of parscale makes far more sense in g3_init_guess than it does in g3_tmb_parscale.