title: function handle to function with multiple output and input variables in filedatastore
body: <p>I am trying to build a fileDatastore from multiple large files using a custom read function.
  This should work like</p>
  <pre><code>fds = fileDatastore(location, 'ReadFcn', @fcn)
  </code></pre>
  <p>I use a cell array containing several fullfile strings for location, which should be fine.
  I simply can't figure out how to handle the read function which has multiple input and output variables and looks like this:</p>
  <pre><code>[A, B, C] = function(filestring, x, y, z)
  </code></pre>
  <p>How can I make this work?
  Thanks in advance!</p>