ENH FileTree:  disallow initializing the same field twice

- use _defaults only for existing keys.
- don't add ghost fields
- add ._find_missing_fields() method
FIX MneExperiment:  .set_epoch()
FIX MneExperiment:  don't initialize any fields twice
FIX MneExperiment:  .make_cov() use proper rejection
ENH FileTree:  increase the depth for auto-making a file
ENH MneExperiment:  `cov-rej` - customize `rej` method for `cov-file`