use more general default selector in multigrid

[yhmtsai]

This PR changes the multigrid selector to more general one. use level as index when level < #mg_level and reuse the last one when level >= #mg_level

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.01%. Comparing base (fb70ec0) to head (5b18510). Report is 359 commits behind head on develop.

:exclamation: Current head 5b18510 differs from pull request most recent head c7ca903

Please upload reports for the commit c7ca903 to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1611 +/- ## =========================================== - Coverage 91.04% 90.01% -1.04% =========================================== Files 700 736 +36 Lines 56996 59831 +2835 =========================================== + Hits 51894 53857 +1963 - Misses 5102 5974 +872 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[sonarcloud[bot]]

Quality Gate failed

Failed conditions
D Maintainability Rating on New Code (required ≥ A)

See analysis details on SonarCloud

Catch issues before they fail your Quality Gate with our IDE extension SonarLint