Blur: remove wrong assert
tuttleHost: graph allows multiple connections between 2 nodes

fix #171
tuttleHost tests: TimeShift offset meaning has changed

* offset direction has changed
* IsIdentity nodes are now correctly removed from the computation, so we can test that.
"forceIdentityNodesCompute" is no more mandatory.
tuttleHost: fix asynchronous access to options
ofxs: minor, clean indentation
tuttleHost test: Edges for tests should have a function "getInAttrName"
tuttleHost test: minor fix
oiio: check if file exists in getClipPreferences