[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

    22 files      22 suites   3d 15h 41m 55s ⏱️
 3 775 tests  3 774 ✅ 0 💤 1 ❌
75 099 runs  75 098 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 9d678bf.