fix: default CMAKE_INSTALL_PREFIX to a sibling folder instead of /opt

[pereiraalessandra]

Summary

• Default CMAKE_INSTALL_PREFIX now points to a sibling folder of the
  source tree (<source>/../neopz_install) instead of the fixed
  /opt/neopz (Linux/macOS) / C:/neopz (Windows).
• Avoids requiring write access to /opt and keeps the installed
  files next to the project folder, which is more convenient when
  working with multiple local NeoPZ builds/projects.
• Users can still override it explicitly with -DCMAKE_INSTALL_PREFIX=....