VNT: Add support for indexing with Arrays of indices to PartialArray

[mhauru]

For indexing into PartialArrays (and hence VarNamedTuples), we currently only support ranges and individual indices, and not things like a[[1, 3]] ~ MvNormal([0.0, 0.0], I), which could be entirely valid things to do. We would like to add support for such array-indexing.

This might be quite an easy fix, but when I was writing the PartialArray code I had an implicit assumption in my mind that any ArrayLikeBlock variables would be contiguous blocks. We would need to read through that code with these non-contiguous blocks in mind and see if anything needs changing, and add tests.