Geofileops

This version contains quite some fixes and small improvements. The most
important ones are listed here.

Improvements

- Stop using sqlite WAL output format as default: gives locking issues when
using files on a file share
- Improve handling of output geometry type
- Add scripts, dependencies,... to use pytest-cov to report unit test coverage
- Add first version of sphynx documentation
- Improve test coverage
- Improve benchmark tests

Bugs fixed

- Negative buffer shouldn't output rows with empty geometries
- Dissolve gave an error in some specific cases
- Join_by_location operation gave wrong results

Most typical operations are now supported: buffer, simplify,
export_by_location, intersect, union, erase, dissolve,...