Introduce possibility to have an iterator per variable

Introduce possibility to have an iterator per variable

This patch introduces the capability to use a different iterator per
variable.

The currently supported notation of variables in a filtering rule like

  <rule action='accept' direction='out'>
     <tcp  srcipaddr='$A' srcportstart='$B'/>
  </rule>

processes the two lists 'A' and 'B' in parallel. This means that A and B
must have the same number of 'N' elements and that 'N' rules will be
instantiated (assuming all tuples from A and B are unique).

In this patch we now introduce the assignment of variables to different
iterators. Therefore a rule like

  <rule action='accept' direction='out'>
     <tcp  srcipaddr='$A[@1]' srcportstart='$B[@2]'/>
  </rule>

will now create every combination of elements in A with elements in B since
A has been assigned to an iterator with Id '1' and B has been assigned to an
iterator with Id '2', thus processing their value independently.

The first rule has an equivalent notation of

  <rule action='accept' direction='out'>
     <tcp  srcipaddr='$A[@0]' srcportstart='$B[@0]'/>
  </rule>