[ntuple] Add ctx manager to RNTupleReader and update RNTupleWriter's

[silverweed]

Now both the Reader and Writer's context objects will close the underlying file upon __exit__. The object will raise an exception if accessed after the with statement.

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    21 files  ±  0      21 suites  ±0   3d 11h 54m 13s ⏱️ + 7h 10m 4s
 3 779 tests +  4   3 778 ✅ +  3  0 💤 ±0  1 ❌ +1 
72 662 runs  +346  72 661 ✅ +345  0 💤 ±0  1 ❌ +1 

For more details on these failures, see this check.

Results for commit b1b0ce6. ± Comparison against base commit 34efbdb.

♻️ This comment has been updated with latest results.