Use CMake file sets to install headers

[tpadioleau]

• Use CMake 3.25 support of HEADERS FILE_SET to use DDC headers in the build and install tree.
• As a consequence this allows to have finer control of which headers are installed, whether fft, splines or the PDI wrapper are enabled.
• Update the install test to detect missing headers.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.02%. Comparing base (97e8db9) to head (587891e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #915   +/-   ##
=======================================
  Coverage   94.02%   94.02%           
=======================================
  Files          55       55           
  Lines        2727     2727           
  Branches      853      853           
=======================================
  Hits         2564     2564           
  Misses         87       87           
  Partials       76       76           

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[tpadioleau]

thanks

[science-enthusiast]

These are not critical. We can address them later if needed.

Earlier we just wanted to error out, now we want to include the file if it is available. The following order is simpler to read:

#if __has_include(<ddc/kernels/fft.hpp>)
#    include <ddc/kernels/fft.hpp>
#else
#    error
#endif

Also, an error message could be added.

[tpadioleau]

I agree I can rearrange them and add a small error message.