tidy: Remove legacy xsec in SplineStucts and const correcntess

[KSkwarczynski]

Pull request description

Plenty of comments were bit dated when splines were only for xsec. This is no longer a case for a while...
Tidy comments
In addition, apply const correctness.

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### Memory and CPU Usage Summary

| Metric  |     Mean    |      Max    |
|---------|-------------|-------------|
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