UVH5 not ideally suited for arrays with relocatable antennas

As described in the current UVH5 memo, the UVH5 format seems to assume/require that antennas have only one position for all the visibilities in the file. Normally data are not taken while the antennas are "on the move" (unless there's a space based VLBI component!), but it may be desirable to support consolidating visibilities from multiple array configurations into one UVH5 file. This is not possible given constraints in the current UVH5 memo (e.g. antenna_positions is limited to Nants_telescope entries).

UVH5 can be used to store visibilities for each array configuration in separate files, each with their own antenna_positions dataset, but the current spec does not allow combining these into one file. Not sure the best way to deal with this, or if it's even worth supporting this.

I guess so long as uvw_array has the correct (u,v,w) coordinates for each baseline-time sample, maybe it doesn't really matter what the antenna positions are?

Start with the current UVH5 memo, focusing on the Nants_telescope limit for antenna_positions and the per-baseline-time uvw_array. Resolve whether consolidated files with changing array configurations should be supported and, if so, define the representation and update the specification accordingly.