Spherical linestring curve

[louisgreiner]

Closes #12

• implement SphericalLineStringCurve methods + tests
• refacto LineStringCurve in PlanarLineStringCurve methods + tests + occurrences

[louisgreiner]

self.max_extend is public but has a getter, whats the point ?

• -> either remove the getter
• -> either change its range to private and add a setter

[louisgreiner]

Should I remove some examples to enhance readability ?
such as example about brest -> nancy, or new_york -> paris

[louisgreiner]

Should I round the coordinates in the tests section ? To enhance readability ?

[Tristramg]

The commits seem a bit a bit overlapping in their functionalities.
I don’t mind if we squash them, as untangling them would be way too complicated compared to the benefits