fix multiline reconstruction (line sewing)
implementation of difference
add style test in ctest (and make it pass)
ban GeometrySet::operator=
union first steps (work in progress)
[union] imcomplete
[union] work in progress
[union] not there yet, but nearly
Merge branch 'dev_union' of github.com:Oslandia/SFCGAL into dev_union

Conflicts:
	src/algorithm/union.cpp