ENH: First attempts to walk in 3d space

Contains two new elements:

volcoords_from_polar is a convenience function
to create 3D-Cartesian coordinates from 3D polar coordinates based on
georef.polar2latlonalt and georef.project

CartesianVolume is a class to interpolate 3-D polar volume data to...

ENH: Added two functions related to construction of N-dimensional grids

ENH: added two convenience functions for quick save/load of arrays and metadata in hdf5 files

ENH: added two convenience functions for pickling and unpickling to module io

DOC: minor edit related to the installation of numpydoc

DOC: added adjustment, verification and export to tutorial_workflow.rst and the corresponding example file.

Added HESSD Technical Note as key reference to title page.

DOC: minor edit in module docstring of adjust.py

ENH: two new functions in util.py related to averaging time series for given time windows.
Functions are average_over_time_windows and average_over_time_windows. Not tested, yet - needs some more work.
Finally, only one of both should survive.

Added some lines to examples/typical_workflow.py related to gridding

Replaced DX sample dataset sample.dx by a new one for Tuerkheim.

Added some content for gridding to the tutorial_workflow.rst

DOC: Added a new tutorial "tutorial_workflow.rst" which provides an overview over a typical processing chain.

This tutorial should address the most common porcessing steps very shortly and
then refer to the corresponding in-depth-tutorials or library reference sections....

DOC: Some edits on the module documentation intro

Some edits to adapt the adjustment_example to the new adjust module

ENH: New adjust module which comes with out-of-the-box cross validation and free choice of Interpolator

DOC: minor changes to documentation in order to realize an adequate Latex built with subsequent pdf compilation

DOC: minor edit in vis

FIX: latitude of origin (central meridian) changes with zone as well as the false easting.

ENH: Added projection string for the RADOLAN stereographic projection to georef.create_projstr