A node is created at the intersection and the corresponding edges
are split accordingly.

Issue: tomalrussel/snkit#15

NB: the file `pyproject.toml` has beend added for testing with local
dev files.

Tests for:
 - multiple intersections
 - partially overlapping lines
 - end-node intersection
 - already split network

NOTE: overlapping lines test doesn't pass!

Issue: tomalrussel/snkit#15

There is now a recursive function extracting the points from
intersections, in an exhaustive way (excluding polygons).

The crossing restriction has been removed because it is wrong
for overlapping segments: the corresponding test now passes.
A notable consequence is the change in handling endpoints:
intersections with endpoints are now accepted.

Issue: tomalrussel/snkit#15

Special treatment of complex lines:
 - self-intersection points need to be found, no built-in way to do so
 - splitting a complex line at a self-intersecting point doesn't do
   what would be expected: a work around when splitting as well.

NB: small fixes from @tomalrussel have been implemented