RXR-2888: skip in case of fit error and add options to reduce precision

[dominic-irx]

This PR now does two things:

1. skips an encounter in case of fit error
2. Adds option to reduce precision of fit, to minimize fit errors (reltol in .fit_options).

[mccarthy-m-g]

@dominic-irx For the .fit_options argument, could we make it take a fit_options() function as input like we're doing for .stats_summ_options (stats_summ_options()) and .vpc_options (vpc_options ()).

The benefit is that we get clearer documentation on what's available, and we get input validation with nice error messages.