PsPlatform check_file_exists always returns True?

Hello @dbeltran, I am debugging the !475 bug using a ps platform running in a Docker container. However, I noticed that even if I move files around with a debugger, the function check_file_exists was always returning True. Looking at the debugger values, looks like that function is only overridden in LocalPlatform, PJMPlatform, and SlurmPlatform. Is that intentional? If so, I guess we should document it in the Platform.check_file_exists function pydocs.