When closing ExternalVector return a view to its data.

[boxdot]

ExtenalVector does not provide indexed access to its data since it may not be yet written to disk. However, after the vector is closed, its data is fully written and constant from that point in time. Often, one wants to access the data after it was closed. So, it would make sense on closing to return an ArrayView to its data for convenience.

What do you think, @imagovrn, @VeaaC?
CC: @eikesr.

[VeaaC]

Agree fully: I would propose to return an ArrayView after closing the vector