Allow skipping freyja boot

[tavareshugo]

Description of feature

To save some computation, it may sometimes be desirable to skip freyja's bootstrapping step.

My suggestion would be to allow using --freyja_repeats 0. At the moment, using it like this throws an internal error from freyja.
But maybe viralrecon could interpret 0 (or any negative number, for that matter) as meaning "don't run the bootstrap step".

In fact, it may be worth considering --freyja_repeats 0 as the default, in case those processing clinical isolates forget to do --skip_freyja and end up spending a lot of unnecessary compute time bootstrapping.

I guess an alternative would be to add a new option --skip_freyja_boot.

[Joon-Klaps]

Good point.
I'll add a skip_freyja_boot and make sure that repeats are >0