Fix visiblity for linux: move explicit instanciations to the header file

[j-rivero]

🦟 Bug fix

Summary

GCC was not happy with the use of a header only types in the explicit instantiations in order to generate the symbols in the library. Fixed by moving them to the header.

Part of the changes for gazebosim/gz-cmake#392

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #565 (0653c5d) into main (beac11e) will not change coverage.
The diff coverage is n/a.

❗ Current head 0653c5d differs from pull request most recent head ec95d7a. Consider uploading reports for the commit ec95d7a to get more accurate results

@@           Coverage Diff           @@
##             main     #565   +/-   ##
=======================================
  Coverage   96.18%   96.18%           
=======================================
  Files         143      143           
  Lines        9826     9826           
=======================================
  Hits         9451     9451           
  Misses        375      375           

Files	Coverage Δ
include/gz/math/MovingWindowFilter.hh	100.00% <ø> (ø)
src/MovingWindowFilter.cc	100.00% <ø> (ø)

[j-rivero]

oh wonderful, there is now a different error in Mac and Windows.

[azeey]

I'm not sure if putting explicit instantiations in a header is the right thing to do (see https://stackoverflow.com/questions/5864401/does-explicit-template-instantiation-go-in-cpp-or-header-file). If every user of this header instantiates these classes, we'd have ODR issues. Also, we'd lose the benefit of explicit instantiation since every translation unit will be instantiating the templates.

[j-rivero]

Current approach won't fix the issue and it triggers build warnings. I close this PR and open a new one.