Rename ".h" headers to ".hpp"

[oparry-ukaea]

Description

Renames all .h headers to .hpp. For consistency, but also because they weren't being picked up by the formatting script.

Type of change

• [x] Bug fix (non-breaking change)

Testing

No new tests

Test Configuration:

OS: Ubuntu 22.04 Compiler: GCC 11.3.0 / OneAPI v2022.1.0 SYCL implementation: Hipsycl v0.9.2 / DPC++ v2022.1.0 MPI details: MPICH v4.0.2 Hardware: CPU (Intel Alder Lake)

Checklist:

• [x] New and existing unit tests pass locally with my changes
• [x] I have run clang-format against my *.hpp and *.cpp changes
• [x] I have run cmake-format against my changes to CMakeLists.txt
• [x] I have performed a self-review of my own code
• [x] My changes generate no new warnings