Refactor: optimized cubic spline

[jinzx10]

Reminder

• Have you linked an issue with this pull request?
• Have you added adequate unit tests and/or case tests for your pull request?
• Have you noticed possible changes of behavior below or in the linked issue?
• Have you explained the changes of codes in core modules of ESolver, HSolver, ElecState, Hamilt, Operator or Psi? (ignore if not applicable)

Linked Issue

This PR is part of #4079

Unit Tests and/or Case Tests for my changes

The relevant unit tests are re-designed.

What's changed?

The cubic spline class is redesigned to enable faster evaluation in the following two situations:

• evaluate a single interpolant at multiple places;
• evaluate multiple interpolants that share the same knots at a single place.

Any changes of core modules? (ignore if not applicable)

No.