Fixing test files for LinkedScene

Links now pointi to the test directory as opposed to files in /tmp and modifying the testWriting() test so that it can recreate the files if necessary.
Adding setPresets() to Parameter class.

This will allow dynamic interfaces to exist, by updating the shown presets based on value changes in other parameters in the same Parameterised object (most likely Ops).
The idea is to make Gaffer support a user hint that forces a given Parameter to be shown as a drop down list control, and then, use the change-value callback mechanism to update the presets and update the current value if not in the new presets.
Adding getPresets() and deprecating presets() function.
Merge pull request #90 from ldmoser/setPresets

Adding setPresets() to Parameter class.
Updating Changes for a15.

Also fixed IECoreGL test environment
Updating IE build options.

Removed dead platforms. Using active Maya versions
Fixing unsigned vs int comparisson
Fixed animated ROP test.

It wasn't actually verifing the results on any time but 0... luckily it was actually working.
Fixed bug with writing re-rooted scenes.

They would throw if caching multiple frames (as shown in the test). Now they correctly check for an existing re-root child before creating a new one.
Fixed compute hack so it only skips compute for outputObjects plug when opening a scene.
Merge pull request #92 from valeriebernard/sceneShapeComputeHackFix

Fixed compute hack so it only skips compute for outputObjects plug
Merge pull request #91 from andrewkaufman/reRootingPatch

Houdini SceneCache ROP Re-rooting patch
Showing a problem with SceneCache SOP loading a PointsPrimitive.

The points are doubling up on each cook, because they aren't linked to any Houdini primitive.
Removing code that wasn't doing anything
Adding a ParticleSystem in ToHoudiniPointsConverter
PointsPrimitive Uniform prim vars are now converted.

Since we have a ParticleSystem for each PointsPrimitive, we can support conversion of the Uniform prim vars to Houdini PrimAttribs.
SceneCache tagFilter supports SceneInterfaces with no tags
Testing HoudiniScene with flattened geometry.

Adding tests showing that it doesn't support flattened geometry (all in one SOP) unless every level of the hierarchy has its own shape.
Testing SceneCache ROP for flattened geometry.

Adding test showing that it doesn't work for flattened geometry, because the HoudiniScene is returning the wrong results.
Fixed HoudiniScene bugs for flattened geometry.

Gaps in the hierarchy are now supported, and this commit fixes the tests from the previous two commits.