Pypiplot

Minor fixes

* Create dir if not exists
* update docstrings

* import changed because class is now with a capital P:
`from pypiplot import Pypiplot`
* added calender plot: `plot_cal()`
* added rolling window plot: `pp.plot()`

* added doi

* Repo data can be stored in path of choice using the savepath parameter.
* Docstrings added/ improved

* First release!