Added flat terain to runatlasdynamics test.
Merge pull request #127 from mposa/master-bugfix

Added flat terain to runatlasdynamics test.
fix for dircol glider bug from ani
more robust path handling (especially for windows)
`getTerrainContactPoints` has collision_group arg
Cache terrain contact points at RBM compile

The results of `getTerrainContactPoints(model)` will not change between
compiles of `model`. Generating that result is slow, since it involves growing
an structure array of unknown length. At compile time, cache the result in a
protected property.
Exclude world when finding terrain contact points

... because world-fixed objects don't collide with the ground.
Populate cpp RigidBody contact_pts

Uses 'cached_terrain_contact_points_struct`. I had to make that property
publicly readable so that `constructModelmex` could access it.
Alternate syntax for collisionDetectTerrain.

Now if a kinsol and terrain contact point struct are passed,
`collisionDetectTerrain` will compute the world location of the contact points
prior to querying `getHeight`. The old syntax (passing in world-frame contact
point locations) is still supported.
Add 'terrain_only' option to collisionDetect

This lets us query just terrain interactions.
Use body_idx in terrain part of collisionDetect

If `active_collision_options.body_idx` is non-empty, only check the bodies
listed against the terrain.
Make collisionDetect[Terrain] work with TaylorVar
Add RigidBodyFlatTerrainNoGeometry class

This is useful for testing whether our collision detection plumbing works for
non-`RigidBodyFlatTerrain` terrains.
Add getTerrainContactPoints to TSRBM
Fix getTerrainContactPoints in RBSphere

Return an empty array for non-zero radius spheres.
Add contactPositionsFromCollisionData

Put computations that were being performed both in `contactConstraints`(in an
optimized way) and in `contactPositions` (in an unoptimized way) into the new
method. Those methods are not updated in this commit, however.
Update contactPositions[Jdot]

These methods now call `contactPositionsFromCollisionData`.
Fix getTerrainContactPoints passthrough in TSRBM
Reset cached_terrain_struct during compile
Replace 0x1 outputs with [] in collisionDetect