BUG: cannot `trace.save()` author model

[eigenfoo]

NotImplementedError: variable 'dim_0' is a MultiIndex, which cannot yet be serialized
to netCDF files (https://github.com/pydata/xarray/issues/1077). Use reset_index()
to convert MultiIndex levels into coordinate variables instead.

cc @aseyboldt

[eigenfoo]

It looks like the simplest solution would be to reset_index before serializing the trace, and then set_index immediately after un-serializing.

This would bring the number of temporary solutions/hacks to 2, to circumvent bugs in xarray. One due to pydata/xarray#2299 and another due to pydata/xarray#1077

@aseyboldt does this sound good to you?

[aseyboldt]

Yes, that sounds good.
To be fair, the first one doesn't really affect us, that just made me waste 15 minutes trying to solve the issue we had with the multiindex.