Add Means of Retrieving Node Names from Extended Network

This commit adds a new member function

    ExtNetwork::node_names() const

which collects the names (std::string) of all nodes in a network.
The immediate use case is creating summary config nodes for network
level quantities specified without explicit lists of nodes, e.g.,

    GPR
    /

Add a simple unit test for demonstration.

opm/parser/eclipse/EclipseState/Schedule/Network/ExtNetwork.hpp

@@ -45,7 +45,7 @@ public:
     const Node& root() const;
     std::vector<Branch> downtree_branches(const std::string& node) const;
     std::optional<Branch> uptree_branch(const std::string& node) const;
-
+    std::vector<std::string> node_names() const;
 
     bool operator==(const ExtNetwork& other) const;
     static ExtNetwork serializeObject();