Add API calls to group and ungroup nodes

## Issue
There is no mechanism to easily group or ungroup sets of nodes. The only interface available is flattenSubGraphs() but this
converts node instances into their nodegraph implementation equivalents which isn't the same.

## Proposal

It would be useful to have a formal API which allows for grouping nodes into a `nodegraph` and ungrouping nodes in a `nodegraph`.

The criteria would include:

1. Preserving input and output connections
2. Removing any unnecessary interface connections on ungrouping.
3. Allow for disconnected grouping (make copy)
4. Preserve any nodegraph attributes which are used to resolve identifiers (such as `fileprefix`).

### Implementation

Add API to `GraphElement`.

Example something like this:
```c++
// Ungroup this nodegraph and return a list of nodes / nodegraph children extracted.
// - inplace means to replace the existing nodegraph
std::vector GraphElement::ungroup(bool inplace=true);

// Group a set of elements (nodes, nodegraph) under a new nodegraph
// - elements : Elements to group
// - graphName : Name of new nodegraph
GraphElement::group(std::vector elements, const std::string& graphName)
```

Start by locating GraphElement and the existing flattenSubGraphs() implementation. Trace how node and nodegraph connections, interfaces, and identifier-resolving attributes are represented; done means adding a formal group/ungroup API that preserves required connections and attributes, supports disconnected grouping, and removes unnecessary interfaces when ungrouping.

Written by the indexing model from the issue text.