Make it easier to set defaults for "format"

[mbercx]

While working on #111, I thinking of implementing an outputs namespace where users can find truly available outputs:

    @cached_property
    def outputs(self) -> SimpleNamespace:
        """
        Namespace populated ONLY with successfully retrievable outputs.
        Access like: `parser.outputs.structure`, `parser.outputs.fermi_energy`, ...
        """
        namespace = SimpleNamespace()

        for name in self.list_outputs():

            try:
                val = self.get_output(name)
            except GlomError:
                continue

            setattr(namespace, name, val)

        return namespace

Neat, but what if they want the outputs in a certain format (e.g. ASE)? Since the outputs are a property, they can't pass a fmt argument here, as with the get_output method. Still would be nice to support this tab-completion approach, though.

Perhaps we can store a default format on class:

pw_out = PwOutput.from_dir('qe_dir', fmt='ase')

And perhaps this default should even be configurable package-wide? Just thought for now.