ENH: First prototype of optional_import function in module util to facilitate lazy loading of optional modules.

This porottype will be refined, soon.

REF: Removed all "from module import *" statements that import funcitons directly into the wradlib namespace.

This requries refactoring of all code that calls wradlib funcitons directly from
the wradlib namespace instead of from the corresponding modules....

MIN: Modification of aggregate_equidistant_tseries in module util.

The function was changed in a way that we now check for each target time interval
whether source and target time specs are consistent. If they are not, the target value
for that target interval will be NaN. Bfore that change, the entire target series...

DOC: Minor documentation edits in dp module overview.

DOC: Fix in dp doc.

DOC: Minor changes in dp documentation.

FIX: changed order of dimension slices. Fixes issue #16

The recipe had not been updated to reflect the change in coordinate dimension
order convention for 3-D data.

FIX: removed assertion to avoid errors with new DX-files.

Apparently the DX-format writers at the DWD have a strange way to calculate
product length. It looks like the 0x03 character used to separate the header
from the data is not counted, or counted once too many or, I don't know....

FIX: pia_from_kdp did not consider gate length when computing PIA from KDP.

FIX: pia_from_kdp did not consider gate length when computing PIA from KDP.

FIX: in correctAttenuationConstrained2-Function, where reference-pia interpolation and pia-calculation in small sectors of incorrect beams only takes place now, if they exist indeed;
- changed internal variabel name invalidbeams -> small_sectors for better understanding...

ENH: made plot_ppi always return axes with equal aspect (avoiding radar 'eggs').

MIN: minor code beautifications.

DOC: added some in-code documentation to vis.plot_ppi and vis.plot_rhi

ENH: added examples for the new plot_ppi routine

NEW: Added function vis.ppi_plot_crosshair

ENH: Added ability to georeference and project coordinates in plot_ppi.

NEW: Introduced functions plot_ppi and plot_rhi.
These functions plot data assumed to be in the radar polar coordinate system
directly on a cartesian matplotlib axes, allowing zooming and easier additions
of other data.

Merged in kai_muehlbauer/wradlib_miub (pull request #13)

Minor changes in cg_plot and GAMIC hdf reader

Merged in thpfaff/wradlib (pull request #12)

ENH: Introduced polar2latlonalt_n and helper functions.