Clarify semantics for the --object-directory option

[whart222]

The gcovr code includes comments that indicates that the --object-directory option should be same one used by gcovr. This is not obvious from the gcovr --help documentation. Additionally, this option is not used by the get_datafiles() function to search for .gc files. That seems like a bug, but I'm not sure.

[latk]

There has been progress on the --object-directory option. Its semantics were studied and simplified in #280. Despite its name, there is no direct relationship between gcov's and gcovr's --object-directory options.

My long-term plan is to make search heuristics user-configurable rather than just offering the choice between standard and --object-directory heuristics.