DS9 regions -> WCS path

Implemented a WCS path and a ds9 region parser that partially depends on my branch of pyregion, at least for segments, but not for lines.

```
In [3]: pvextractor.paths_from_regfile('tests.reg')
Out[3]:
[<pvextractor.geometry.path.WCSPath at 0x10e4861d0>,
 <pvextractor.geometry.path.WCSPath at 0x10e44fa90>]
```

The only two that work are segment and line:
```
# Region file format: DS9 version 4.1
global color=green dashlist=8 3 width=1 font="helvetica 10 normal roman" select=1 highlite=1 dash=0 fixed=0 edit=1 move=1 delete=1 include=1 source=1
galactic
# segment(49.506329,-0.1652525,49.488441,-0.3179075,49.367701,-0.33149222,49.321696,-0.42449278)
line(49.048054,-0.36365278,49.185208,-0.29674361) # line=0 0
# vector(48.944348,-0.36432694,485.647",124.082) vector=1
panda(49.225356,-0.35293833,327.834,380.483,1,120.384",240.769",1)
# composite(48.841754,-0.26420167,1.82386e-10) || composite=1
point(48.894165,-0.28070194) || # point=x
point(48.848667,-0.25728806) || # point=x
point(48.78243,-0.25461444) # point=x
# projection(49.048054,-0.36365278,49.185208,-0.29674361,1")
```