Refactor geometry

Example code to use this so far:

```
from astropy.io import fits

from pvextractor import pv_slice

x = [50., 0.]
y = [50., 0.]

data = fits.open('L1448_13CO.fits')[0].data

slice = pv_slice(data, x, y)

fits.writeto('slice.fits', slice, clobber=True)

slice = pv_slice(data, x, y, interpolation='nearest')

fits.writeto('slice_near.fits', slice, clobber=True)

thick_slice = pv_slice(data, x, y, width=5)

fits.writeto('slice_thick.fits', thick_slice, clobber=True)
```