Fixed LinkedScene write bug, added test case

LinkedScene was erroring on write if you wrote a link, then wrote some transform samples into the link whose first time didn't coincide with the link's first bound time.
sceneInterface:link has now been broken down into sceneInterface:root, sceneInterface:fileName and sceneInterface:time in the LinkedScene, although the mechanism for writing a retimed link remains the same
changed the names of the link/time attributes in the LinkedScene
totally disabled time remapping for legacy linked scenes, plus fixed a typo/bug
fixed first time write bug
Merge branch 'linkedSceneLinkAttributes' of https://github.com/davidsminor/cortex into linkedSceneLinkAttributes

Conflicts:
	test/IECore/LinkedSceneTest.py