C++17 code found in ylm.cpp, need to eliminate it, we only support up to C++11

[mohanchen]

Describe the Code Quality Issue

In file included from /home/510Group/2_abacus/abacus-mc/source/source_base/ylm.cpp:10:
/home/510Group/2_abacus/abacus-mc/source/source_base/ylmcoef.h:15:1: warning: inline variables are only available with ‘-std=c++17’ or ‘-std=gnu++17’
15 | inline constexpr double ylmcoef[YLMCOEF_SIZE] = {
| ^~~~~~

Additional Context

No response

Task list for Issue attackers (only for developers)

• Identify the specific code file or section with the code quality issue.
• Investigate the issue and determine the root cause.
• Research best practices and potential solutions for the identified issue.
• Refactor the code to improve code quality, following the suggested solution.
• Ensure the refactored code adheres to the project's coding standards.
• Test the refactored code to ensure it functions as expected.
• Update any relevant documentation, if necessary.
• Submit a pull request with the refactored code and a description of the changes made.