Added migration plan

Author: tomdurrant

MIGRATION_PLAN.md

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+# 🔄 Migration Notice: Repository Restructuring for `rompy`
+
+The `rompy` project is being restructured to improve clarity, modularity, and long-term maintainability.
+
+We are splitting the monolithic repository into four smaller, purpose-driven repositories:
+
+---
+
+## 📦 New Repository Layout
+
+| Repository | Purpose |
+|-----------|---------|
+| [`rompy`](https://github.com/rom-py/rompy) | 🧠 Core framework (remains at original location) |
+| [`rompy-swan`](https://github.com/rom-py/rompy-swan) | 🌊 SWAN model components |
+| [`rompy-schism`](https://github.com/rom-py/rompy-schism) | 🌊 SCHISM model components |
+| [`rompy-notebooks`](https://github.com/rom-py/rompy-notebooks) | 📓 Examples, tutorials, workflows |
+
+---
+
+## 🗂️ Migration Plan
+
+1. **Rename the current `rompy` repository**  
+   - The existing monolithic repo will be renamed to `rompy-legacy`.
+   - All code, history, issues, and PRs will remain in `rompy-legacy`.
+
+2. **Create a new, slimmed-down `rompy` repo**  
+   - A new repository named `rompy` will be created at the original URL.
+   - It will contain only the clean, minimal core library.
+
+3. **Spin out model integrations and notebooks**  
+   - SWAN-related code → `rompy-swan`  
+   - SCHISM-related code → `rompy-schism`  
+   - Tutorials/examples → `rompy-notebooks`
+
+4. **Cross-link all repos**  
+   - Each repo will contain a README banner pointing to the others.
+
+---
+
+## 🔁 GitHub Redirect Behavior
+
+GitHub automatically sets up redirects when a repository is renamed:
+
+- **Existing clones of `rompy` will transparently point to `rompy-legacy`.**
+- **Pulls and pushes will be redirected to `rompy-legacy` — safely.**
+- **No one will accidentally overwrite the new `rompy` repo.**
+- Issues, PRs, commit hashes, and tags remain valid under the new name.
+
+> ✅ **You do *not* need to change your local git remotes.**  
+> Your clone of `rompy` will now be pointing to `rompy-legacy` automatically.
+
+> ⚠️ **Do not update your `origin` to the new `rompy` repo unless you explicitly want to work on the new core.**  
+> Otherwise, you may push unrelated history to the new slimmed-down repo.
+
+---
+
+## 🧠 Why This Approach?
+
+### ✅ Safe for existing users
+- Users with existing checkouts keep working on the full-featured legacy repo.
+- They don’t need to change anything.
+- Pushes/pulls still work — but go to `rompy-legacy`.
+
+### 🧭 Clear separation of concerns
+- `rompy` → clean, minimal, core logic
+- `rompy-swan` → SWAN model components
+- `rompy-schism` → SCHISM model components
+- `rompy-notebooks` → examples and workflows
+
+### 🧬 Preserves history
+- Full development history is retained in `rompy-legacy`.
+- New repos  retain relevant history using `git filter-repo`.
+
+### 🛠 Easier maintenance
+- Each repository can evolve independently.
+- CI, releases, and issues are more focused.
+- Contributors can work on what matters to them without navigating an oversized codebase.
+
+---
+
+## 🧑‍💻 For Contributors
+
+If you want to:
+- 🧾 **Continue working with the full legacy code** → keep using your current clone. It now points to `rompy-legacy`.
+- 🧠 **Start working on the new slimmed-down core** → clone the new `rompy`:
+  ```bash
+  git clone https://github.com/rom-py/rompy.git
+