Prevent mulitple contract addresses on the same chain (for now)

[SmartLayer]

Handling of multi-address contracts. We decided for Method 2.

 Method 1 - (original)
 ----------------------------------------------------------
 <contract name="holding">
 <address network="1">xxx</address>
 <address network="1">xyy</address>
 </contract>

 <contract name="oracle">
 <address network="1">yyy</address>
 <address network="1">yxx</address>
 <!-- error: non-holding contracts can't have more than 1 address on a network -->
 </contract>

 Method 2 - (we decided for this)
 ----------------------------------------------------------
 <contract name="holding">
 <address network="1">xxx</address>
 <address network="1">xyy</address>
 <!-- error: each contracts can have only 1 address on a network -->
 </contract>

 <contract name="oracle">
 <address network="1">yyy</address>
 <address network="1">yxx</address>
 <!-- error: each contracts can have only 1 address on a network -->
 </contract>