Add engine.convert Ops to go from primitive/boxed arrays to FinalDimensions

Consider Ops like these two. It'd be nice (especially from Python) to be able to write ops.op("create.img").input(new long[] {50, 50}, new FloatType()).apply(). Right now, you won't match the first Op because you have a long[], not a Dimensions. You won't match the second because you provided an element type.

Should be possible to convert a long[] to a FinalDimensions such that we can use conversion to call the first Op.

cc @elevans