Supporting undirected CSR graphs

## Problem

Currently, the Boost Graph Library's `compressed_sparse_row_graph` only supports directed and bidirectional graphs. There are explicit comments in the codebase and documentation indicating that undirected CSR support is a planned but unimplemented feature:

- [`compressed_sparse_row_graph.hpp`](https://github.com/boostorg/graph/blob/3131c24630e42c79b43c1f32558041c219ab84b8/include/boost/graph/compressed_sparse_row_graph.hpp#L239-L259):
> At this time, the compressed sparse row graph can only be used to create directed and bidirectional graphs. In the future, undirected CSR graphs will also be supported.
- [`doc/compressed_sparse_row.html`](https://github.com/boostorg/graph/blob/3131c24630e42c79b43c1f32558041c219ab84b8/doc/compressed_sparse_row.html#L316-L337):
> At this time, the CSR graph type only supports directed and bidirectional graphs...

This is problematic for several reasons, e.g. for adjusting performance in my Louvain's implementation algorithm: I can't use Louvain on directed graphs, but I can't use undirected CSR. Consequently a gap exists between BGL implementations and GenLouvain (that uses CSR). Having an undirected CSR could bridge that gap:

## Feature Request

Add support for undirected compressed sparse row (CSR) graphs to `compressed_sparse_row_graph`. This would improve memory efficiency and performance for large undirected graphs, matching the benefits provided for directed graphs.