Update math_structures.h

[lcyyork]

Description

Intel 2025.0 complains about the insert method in VectorSpaceList, which is only defined in VectorSpace. I think the correct way is to use add instead of insert for VectorSpaceList. Please close this PR if I am wrong. Thanks!

User Notes

• [ ] Features added
• [ ] Changes to compilation (if any)

Checklist

• [ ] Added/updated tests of new features and included a reference output.ref file
• [ ] Removed comments in code and input files
• [ ] Documented source code
• [ ] Checked for redundant headers
• [ ] Checked for consistency in the formatting of the output file
• [ ] Documented new features in the manual
• [x] Ready to go!

[fevangelista]

The compiler is right. This was never detected because the function is probably not used (and therefore not compiled).