Added some docstrings to `plotypus.lightcurve` and `plotypus.utils`.
Added the function `plotypus.lightcurve.get_lightcurves_from_file`.
Modified `plotypus.utils.pmap`, so that the default `callback` function
can take keyword arguments as well, and made use of the `with` statement
in creating the `Pool` object, to ensure proper closing of the object.