Remove `workdir` parameter in config file

[singularitti]

Now we have workdir as a parameter and has it checked. But it does not seem to be necessary. The directory where the config file sits should be considered as workdir, as it is currently used by default. Let's see why:

1. The templates are still need to be provided paths, and they are not necessarily relative to workdir.
2. xps command, or using from Julia REPL, has to specify a config file path, and that was supposed could be not in workdir. But now users have to set path for config files, workdirs & templates, that's a lot of complications.
   So I am considering just deprecate the workdir parameter.