`StructuredData.to_binary` assumes there is only a single data set (which is named `data_array`)

[agzimmerman]

I stumbled on this now while trying to push volume data to Liquid Earth. The StructuredData has five data sets named Features and Properties, X, Y, and Z. I talked to @Leguark and the bug looks obvious enough (as summarized in this title). For now I will proceed without modifying Subsurface because I can create a new StructuredData with only one data array (e.g. data_array = ss.StructuredData.from_numpy(geo_model.solutions.s_regular_grid.data.property_matrix.data) and achieve my current goal.