Break the dependency between an imported group and its parent.

Per correspondence with Jonathan the dependency serves no
useful purpose. It also prevents safely deleting groups preceding
imported groups.

src/file.cpp

@@ -451,6 +451,11 @@ bool SolveSpaceUI::LoadFromFile(const std::string &filename) {
             char *key = line, *val = e+1;
             LoadUsingTable(key, val);
         } else if(strcmp(line, "AddGroup")==0) {
+            // legacy files have a spurious dependency between imported groups
+            // and their parent groups, remove
+            if(sv.g.type == Group::IMPORTED)
+                sv.g.opA.v = 0;
+
             SK.group.Add(&(sv.g));
             sv.g = {};
             sv.g.scale = 1; // default is 1, not 0; so legacy files need this

src/group.cpp

@@ -186,7 +186,6 @@ void Group::MenuGroup(int id) {
 
         case GraphicsWindow::MNU_GROUP_IMPORT: {
             g.type = IMPORTED;
-            g.opA = SS.GW.activeGroup;
             if(g.impFile.empty()) {
                 if(!GetOpenFile(g.impFile, "", SLVS_PATTERN)) return;
             }