Allow read-only directories with multiple references for the same field

For directories that are only read, it's sometimes useful to have a reference field that has several definitions to external directories.

For instance the "groups" information of the user directory could come from two different ways of defining groups.

This is something that works only for read though, because on write we wouldn't be able to know to which reference the write should be done.

So with this feature a directory field can have several references:

<references>
  <!-- we define two references for the "groups" field -->
  <tableReference field="groups" directory="groupDirectory"
    table="user2group" sourceColumn="userId" targetColumn="groupId"
    dataFile="test-user2group.csv" />
  <tableReference field="groups" directory="otherGroupDirectory"
    table="user2othergroup" sourceColumn="userId" targetColumn="groupId"
    dataFile="test-user2othergroup.csv" />
</references>