add "px" to the units allowed for --rat-thickness

[KaiMartin]

If the rat lines are supposed to be drawn with a fixed width of screen pixels, current versions of pcb require --rat-thickness to be given as nanometers with a value below 20nm. This patch adds "px" to the set of allowed units. Internally, the property is still treated as nanometers.

To reduce the probability of suprise, the patch sets the size that decides between interpretation as pixels and a scalable size to 100. Before the cmil/nm conversion, the limit was 20 cmil. So 100nm is still a few orders of magnitude smaller.

---<)kaimartin(>---

[Andrew Poelstra (asp11)]

Thanks Kai! I think pcb looks much more professional (well, more like Altium) with fixed-size rats, and it's great to have the option.

I've commited your patch as 8b59dba1.