ENH: Add normalize() method

[jorisvandenbossche]

Closes #994

This exposes GEOSNormalize as a method on the geometry classes. In GEOS, this works in place, but to keep the Shapely interface consistent (other operations like simplify also return a new geometry), I made the normalize() method to return a new geometry (and thus first clone the geometry before normalizing it).

This seems to work for me for some simple cases. cc @tomplex @Lucidiot

Especially given the overlay changes in GEOS 3.9.0 (where you often first need to normalize the geometry if you want to be able to compare it to an expected result across different GEOS versions), I think it would be useful to still include this in Shapely 1.8.

[mwtoews]

This feature should be welcome for 1.8.

[coveralls]

Coverage increased (+0.01%) to 85.14% when pulling b269b39 on jorisvandenbossche:normalize into 1c56128 on Toblerity:master.

[mwtoews]

Should normalize should be a method property or regular method? E.g.:

• Method property: line.normalize returns a geometry
• Regular method: line.normalize() returns a geometry

I think a method property might be more fitting, since there are no arguments, and there are already many other method properties (e.g.) centroid returns a geometry.

[tomplex]

I think centroid makes sense as a "property" because it's a geometric attribute of the geometry, whereas normalize is an operation on the geometry which is doing something to it, so a traditional method call feels right to me. Just my two cents though.

[sgillies]

Approve! Thank you @jorisvandenbossche !