Make `wire_load_tree_` default to unknown so we can detect when it's not present in a Liberty file. (#402)

This is potentially a behavior change, but I think omitting this is rare. I've only seen it in some DTCD Liberty files.
In those Liberty libraries, it seems to be expected that the Liberty data is valid for all `WireloadTree` types.
Thus it is necessary to distinguish between "wire load tree was specified as 'balanced'" and "wire load tree
was not specified".

liberty/Liberty.cc

@@ -3042,7 +3042,7 @@ OperatingConditions::OperatingConditions(const char *name) :
   Pvt(0.0, 0.0, 0.0),
   name_(name),
   // Default wireload tree.
-  wire_load_tree_(WireloadTree::balanced)
+  wire_load_tree_(WireloadTree::unknown)
 {
 }
 

parasitics/Parasitics.cc

@@ -257,12 +257,12 @@ Parasitics::makeWireloadNetwork(const Pin *drvr_pin,
         makeWireloadNetworkWorst(parasitic, drvr_pin, net, wireload_cap,
                                  wireload_res, fanout);
         break;
+      case WireloadTree::unknown:
       case WireloadTree::balanced:
         makeWireloadNetworkBalanced(parasitic, drvr_pin, wireload_cap, wireload_res,
                                     fanout);
         break;
       case WireloadTree::best_case:
-      case WireloadTree::unknown:
         makeWireloadNetworkBest(parasitic, drvr_pin, wireload_cap, wireload_res,
                                 fanout);
         break;