reworked AbstraceMappedDateParameter

luigi.Parameter containes a class attribute `__default`;
since we needed to map the default value provided by the user,
we needed to modify `Parameter.default`, which in turn uses
`self.__default`. A quick hack, which solves this problem,
is to just duplicate those methods from `luigi.Parameter`, that
reference `self.__default`: `__init__`, `default`, `set_default`
and `has_default`. TODO: find some more lightway way to subclass
`luigi.Parameter`.
fix method formatting