Rename InferenceType to InferenceVariable

[dehann]

Likely only Julia is affected by renaming. <: IIF.AbstractVariableType should replace <:IIF.InferenceType.

[dehann]

This fits better with counterpart AbstractFactor

[dehann]

I'm using AbstractVariableType rather than AbstractVariable because everyone in Julia is building Variable-something, so just helps avoid name conflicts (pre-emptively mind you).

[dehann]

Ah, was already changed to DFG.InferenceVariable

[dehann]

This also avoids likely name conflicts as mentioned in comment #809