Clang tidy c++ files

This fixes all of the clang-tidy problems in tng_io.cpp and md5.cpp but does not yet have the CMake changes in !33 (merged) rebased on top.

I hope this is what you meant by profit @acmnpv apologies if not.

Notes:

The majority of the new explicit casts were applied automatically by clang-tidy --fix. Some that have been marked //NOLINT I was a bit unsure about.