This adds a new operation interface `SDFGConvertibleOpInterface` that
allows an operation to specify how it is converted to an SDFG
process. The interface consists of a single method `convert` that
receives as the arguments the DFG created using `SDFG.init`, a set of
SDFG input streams corresponding to the operands and a set of output
streams for results. The order of the input and output streams
corresponds to the order of the operands and output values,
respectively.
