Link: Non-linear constraints

Link: Non-linear constraints

Add non-linear constraints to links.
Similar to disks, user can use a callback to dynamically change the
link's capacity based on the number of flows sharing it.
Only traffic explicit generated by the user is considered as active
flow, so cross-traffic communications aren't taken into account.

Extend s4u::Link::sharing_policy
- New type: NONLINEAR

Example: examples/cpp/network-nonlinear/