Binary export extension

I use JLD2 for saving my simulation results. The two most important pieces that I save are solution vectors and a `DofHandler`. The only annoying part is if structs change they can't be reloaded that easily. However, JLD2 offers a lot of API to cover that by e.g. [custom serialization](https://juliaio.github.io/JLD2.jl/dev/customserialization/) and [type remapping](https://juliaio.github.io/JLD2.jl/dev/advanced/#Explicit-Type-Remapping)

What do you think about providing a binary export extension for e.g. JLD2 where we specify a custom serialization for the `DofHandler`?