Use BOOST_OVERRIDE on functions that override

[bmagistro]

When compiling with clang 19.1.7 on Alma with -Werror and a number of other flags (we run with most warnings enabled), a number of functions are flagged as overriding a function but not marked with override eg error: 'parse' overrides a member function but is not marked 'override' [-Werror,-Wsuggest-override]. Based on older tickets, the BOOST_OVERRIDE macro was used to address this.

Env:
Alma 9.6
Boost 1.88.0 via cmake
clang 19.1.7

Replaces: #97
While ^^ addressed other issues, to keep scope and review simple, the other issues are not addressed in here.

Resolves: #93

[vprus]

Thanks for the PR. Would you be willing to modify it to use 'override' directly, without any macros, since there's no point supporting C++03.

[bmagistro]

will certainly take a look. this approach was chosen by looking at how it was approached in this/other components