Make persistence of graph format of layered graphs a choice

With https://github.com/innvariant/deepstruct/blob/9401df9896870d7cd8d45bfba8da3754da2f6950/deepstruct/graph.py#L8C11-L8C11 layered graph is currently always stored as graphml file, which is in the sizes of megabytes instead of few bytes compared to adjacency lists. There should be a different choice or a parameterization for the format