title: Scala use fill or tabulate with mutable entries
body: <p>The following generates a Vector of immutable Vectors:</p>
  
  <pre><code>var darr = Vector.tabulate(2, 3){ (a,b) =&gt; a*2+b }
  darr: scala.collection.immutable.Vector[scala.collection.immutable.Vector[Int]] = Vector(Vector(0, 1, 2), Vector(2, 3, 4))
  </code></pre>
  
  <p>But what is needed for our use case is a Vector of <em>mutable</em> Vectors.  How can that be done?</p>