Standardise Cppcheck suppressions vs inlines

[jhaigh0]

Describe the outcome that is desired.
Document the following:

• The Cppcheck suppressions file (CppCheck_Suppressions.txt.in) should be used for cpp check issues that are waiting to be reviewed.
• If the issue is genuine and fixable, fix the problem and remove the line from the file.
• If the issue is unavoidable / a problem with Cppcheck, move to an inline suppression comment and delete the line from the file.