Clean up around BsplineAllocator.hpp and MultiSpline.hpp

[ye-luo]

Proposed changes

1. reduce MultiSpline::create_spline template parameters
2. Make MultiSpline::copy_spline a free function.

What type(s) of changes does this code introduce?

• Other (please describe): code refactoring.

Does this introduce a breaking change?

• No

What systems has this change been tested on?

epyc-server

Checklist

• Yes. This PR is up to date with current the current state of 'develop'
• Yes. Code added or changed in the PR has been clang-formatted

[ye-luo]

Test this please

[ye-luo]

Test this please

[PDoakORNL]

This basically looks good to me, in that it improves the existing c code. I think that from a compilation standpoint copy_spline MSDT & SSDT type mixing is more open than the member function copies, if their are constraints those should be documented and perhaps handled at compile time.

[ye-luo]

Actually I still need to relocate copy_splines. Is that OK if I add "MSDT & SSDT" constraints in my next PR? I will add static_assert making sure they are float types.

[ye-luo]

Test this please