Access NGV Data from a Basalt graph

I would like to access the `ngv_connectome` and `ngv_data` from basalt using an interface similar to the ones in `basalt/ngv`. Following our discussion with @tristan0x we agreed to ignore feature based information retrieval for basalt and use hdf5 for that in the steps of the pipeline that information is required in bulk by feature. Information that is retrieved per astrocyte / synapse / neuron / Vasculature Segment is more suitable for basalt.

So far, I was first querying the `NGVConnectome` in order to retrieve the indices for the neighbours, and then extracted their respective data from `NGVData`. This has to change given the basalt organization where the data is stored in the nodes.

The goal is to be able to query basalt for an astrocyte id and retrieve its information and neighbors. For example, the information required for one astrocyte for the morphology synthesis is the following:

* soma_position, radius
* microdomain geometry (vertex_coordinates, triangles )
* neuroglial_connectivity's synapses assigned to astrocyte's microdomain
* gliovascular_connectivity's endfeet surface targets

I think it would make sense to first focus on the astrocyte perspective.