Couple of code compliance issues

[jcfreeman2]

Running dbt-build.sh with the --lint argument as well as looking at some of the code, there are some improvements which could be made:

• A lot of the files in trigger don't have our standard copyright message
• As a collection of public data, Set.hpp is more properly a struct than a class
• In general, no-op virtual destructor definitions ("virtual ~MyClass() {}") would be better declared default ("virtual ~MyClass() = default") if for no other reason than to not fool the linter into thinking the destructor is nontrivial (for more though, see here)

Other minor issues (and a couple of false positives, unfortunately) can also be unearthed via --lint

[ArturSztuc]

@jcfreeman2 I wanted to have a look into this and close this old issue, but dbt-build --lint doesn't work for me as expected:
It does print the Package to lint is ... messages, and it generates the linting__DATE__ folder with logs for individual packages, but they are all empty.