New XML file structure

See merge request !93

- Simulation = { World, Policies, [Agents] }, World = { [Obstacles] }.
- CrowdSimulator: Changed XML loading.
- Changed all example files.
- Adding some missing example files.

Merge obstacles into dev_whatever

See merge request !92

- Implemented obstacle avoidance force.
- Renamed parameters to match the Helbing/Molnar paper, and explained them a bit better in the documentation.

- Added the method ComputeObstacleInteractionForce() to the two subclasses, PowerLaw and SocialForcesAvoidance. Currently, these methods return the zero vector.

Updated Doxygen comments for cost functions. Also added a warning whenever a cost function does not yet support obstacles.

TtcaDca::GetGradient(): Fixed a bug, which suggests that the math of the "Dutra gradient" is really incorrect. Now using the approximation instead.

Started improving ForceBasedFunction so that it translates better to the...

See merge request !91

Started improving ForceBasedFunction so that it translates better to the velocity domain. GetGradient(currentvelocity) still returns the same vector as before, but the costs and gradients are now more meaningful for *other* velocities.

- Regular sampling can now use the base velocity as a sample as well. (This setting is switched off by default.)
- Updated the Karamouzas policy to use this new setting.

Resolve "Find legal way to use PowerLaw"

See merge request !90

- Replaced examples by the ones used more recently for UMANS.

This should be a better (yet still simple) approximation of the gradient.

Resolve "Add ORCA, with correct licensing"

See merge request !89

- Changed the ORCA solver and cost function accordingly.

- Implemented ORCA::GetCost() using those cached results.
- Added ORCA folder to CMakeLists.txt.

- CostFunction: Bugfix in time-to-collision with line segments.