Add comparison operators to the standalone ElementLink

The standalone ElementLink implementation gains implicit comparison operators through its implicit conversion to ElementConstPointer. However this is ultimately a pointer to an internal cached pointer, so even if you construct two identical element links one will compare less than the other (even though they will compare equal using the equality operator).

This came up where links are used as keys in a map here and causes issues seen in reading the navigation in AnalysisBase.

This MR introduces comparison operators that match the operators in Athena. The body is a lot simpler than in Athena because the standalone element links do not default to -1.

This should resolve the trigger matching issues seen in ATLMCPROD-10089 and ATR-26071