implement opaque types

[magnusuMET]

Add two new functions to Variable for putting and getting raw bytes

[lwandrebeck]

There’s a WIP in https://github.com/lwandrebeck/rust-netcdf/commit/c69077643b1eb782f1a3eb7a43907058dfd657f4
Feel free to work on it a bit more, I probably won’t have much time in the coming days.

[magnusuMET]

I think this might require quite a lot of work when it comes to exposing netcdf types as some sort of rust structure. I'm going to remove the "good first issue" tag.

The best thing would be to have a serde like approach to serialising data, where one can put a #derive(netcdf) on a struct, but I haven't really had a look at how that is done.

[magnusuMET]

We need to add a Vec or HashMap to store the types in each Group, and some way of using the generic Numeric trait on the custom types.

[magnusuMET]

#37 allows us to traverse the tree upwards to get any types. Each variable now needs a weakref to the group, and we can find the applicable type.