Fine-tuning for FileNameOption.

Fine-tuning for FileNameOption.

- Change behavior of required options with default values such that the
  default value is now used without an error if the option is not
  provided.  This is how it used to work with file name options.  For
  most other cases, this combination doesn't make much sense, but
  changed it such that it works consistently for all cases.
- Add FileNameOption::defaultBasename() as a more intuitive interface
  for providing a default for file name options.  The name makes it
  clear that no extension is expected, and the option also transparently
  treats required/optional arguments without the user needing to
  understand the difference between defaultValue() and
  defaultValueIfSet().

Closes #642; there is still some things to be improved, but should now
provide about the same features and about the same input options as
the old t_filenm-based system.

Change-Id: Ibcaf0667180e0b24b08604869d855baf23476681