Remove some rates settings from module-level variables

[jschwab]

The call to rates_init sets some module-level configuration variables (e.g., rates_table_dir, use_special_weak_rates). The last call to rates_init will "win" which can lead to some surprising outcomes if you pick different values for these in each star of a binary. So you basically get forced to use the same settings for both stars (whether you know it or not...).

In eos/kap/net we have a handle and General_Info structure where we can store config options like that (and then different stars have different handles), but there's no such thing in rates. Not clear to me how to do this cleanly in the current structure.

This is quite a longstanding limitation, but someday if rates/net get cleaned up, it should be fixed.