Update FDD_MPE to allow for Nref=1

[keltouny]

Allow for a single reference channel in the Multi-step FDD algorithm.

[dfm88]

Dear @keltouny,

thank you for your PR, while we are reviewing this, I want to inform you that we've recently performed maintenance on the repository to remove large binary files from the Git history, which required a history rewrite. This means all branches now have a new history.

Action required: Since this PR was created before the history rewrite, it will likely have conflicts that cannot be resolved normally. You'll need to:

1. Update your local repository:

git fetch origin
# or any branch you are working at
git checkout main
# stash your local changes first
git reset --hard origin/main

2. Recreate your feature branch:

git checkout -b your-feature-branch-name

3. Reapply your changes (either manually or using a patch if you saved one)

4. Push the new branch and open a new PR

Unfortunately, Git doesn't provide a clean way to automatically fix PRs after a repository-wide history rewrite. We apologize for the inconvenience, but this maintenance was necessary to reduce the repository size and improve clone times for all users.

Please let us know if you need any assistance with this process.

[dagghe]

Dear @keltouny could you please update your pr so that we can include it