Add DCT implementation

Description

Add DCT implementation, plus tests and benchmarks, for transforms of size with no factors other than 2, 3, 5 and 7 (all others currently return unsupported).

Checklist

• Contribution meets RAL's licence terms
• New functions adhere to RAL's naming scheme
• Contribution conforms to RAL's directory structure
• Documentation updated
• [] "Unreleased" section of the Changelog updated - I will do this in a follow-on patch
• clang-format and clang-tidy run and changes included (C/C++ code)
• flake8 run and changes included (Python code)
• cmake-format run and changes included (CMake code)
• Tests added or updated
• Tests pass when run with AddressSanitizer
• Benchmarks added or updated