title: Specify concat_dim for xarray open_mfdataset
body: <p>I'm using <code>xarray.open_mfdataset()</code> to open and combine 8 netcdf files (output from model simulations with different settings) without loading them into memory. This works great if I specify <code>concat_dim='run_number'</code>, which adds <code>run_number</code> as a dimension without coordinates and just fills it with values from 0 to 7.</p>
  
  <p>The problem is that now, I don't know which run_number belongs to which simulation. The original netcdf's all have attributes that help me to distinguish them, e.g. <code>identifyer=1</code>, <code>identifyer=2</code>, etc., but this is not recognized by xarray, even if I specify <code>concat_dim='identifyer'</code> (perhaps because there are many attributes?).</p>
  
  <p>Is there any way in which I can tell xarray that it has to use this attribute as <code>concat_dim</code>? Or alternatively, in which order does xarray read the input files, so that I can infer which value of the new dimension corresponds to which simulation?</p>