Use .hpp extension for headers

[tylerjw]

https://github.com/cpp-best-practices/cppbestpractices/blob/master/03-Style.md#use-hpp-and-cpp-for-your-file-extensions

[JorgePerC]

Hey, I would like to help, are there any specific instructions on how to do so?

[TSNoble]

I'd be interested in picking this up if it's not being worked on?

[TSNoble]

Should be finished with the following items:

• Replace majority of internal headers with hpps (Enhancement/use hpp for headers #3113)
• Replace c library imports with c++ equivalents
• Replace ros.h imports with rclcpp imports
• Replace ROS1 msg imports with ROS2 msg imports
• Replace fmtlib imports with std::format
• Replace non-ROS library imports with hpp imports where possible
• Fix header install destination (Fix header install destination #3134)
• Replace ikfast.h with ikfast.hpp (may be fixed by autogeneration of deprecated c-style headers)
• Get GENERATE_EXPORT_HEADER() macro to export hpps
• Linting to ensure that the standard is maintained (i.e. fail on detection of .h files)

Edit: Added the points below to the list. Will mark as outdated when I'm back on my work account