Support fetching separate realisations as ensemble-wide

[pinkwah]

We currently support "upcasting" a user GETing a single realisation out of an ensemble-wide record transparently. However, if the user uploads each realisation separately, fetching it as ensemble-wide does not work and gives a 404.

It should be supported, but thinking about it, I think the solution would need to do a bit of refactoring to get it to work.

[mortalisk]

Shouldn't we just have a concept of having a matrix? And then also be able to both upload and download either the whole, or parts of it, as needed? Do we need a concept of "ensemble-wide"? Do we need to care about whether it is related to ensembles at all?