ENH: Added function issequence to wradlib.util

Tests whether input is an iterable sequence of numbers.

DOC: Changed "Ream" section to "Community" and added section "Recipes".

In the Community section, there is now a decription on how to carry out Pull Requests.
The section "Recipes" is intended to provide cookbooks/recipes with a lower level of
documentation as the Tutorials section. Just a breif description, a sample output...

DOC: Added PyTables to the dependencies list.

FIX: Removed a condition which tests the validity of ODIM_H5 files.

The condition is to strong and should be replaced by a fancier check in the future.

FIX: setup.py shjould also work with the install --user option.

All files from the directories which are *not* the package (i.e. wradlib)
directories are considered *data* files. In order to make sure these files
are installed to the same site-package directory as the wradlib module files, we...

FIX: minor edit in wradlib.util.sum_over_time_windows to make sure that accumulation continues in case of empty time window.

FIX: Make sure that obs and est arrays in wradlib.verify.ErrorMetrics have the same length.

FIX: read_EDGE_netcdf is now more flexible with respect to azimuth angle resolution

read_EDGE_netcdf now accepts non-equidistant azimuth angles, but also introduces
an option in order to enforce equidistant azimuth angles (by setting
keyword enforce_equidist=True)...

ENH: added another experimental function for aggregating time series data

In this case, sum_over_time_windows computes the sum within specificied time
windows if a minimum of time steps of the original time series is available and valid.

Another helper function _get_tdelta was added, too.

DOC: just a typo in module vpr documentation

ENH: CartesianVolume is now a parent class for two inherited classes CAPPI and PseudoCAPPI

Also added lots of documentation in module vpr.py related to these classes.
Added a refrence to vpr module in tutorial_workflow.rst.

Merged from thpfaff/wradlib (pull request #5)

ENH: the class CartesianVolume does now allow for the creation of Pseudo CAPPIs

However, the solution is preliminary (using a Boolean flag argument named pseudocappi). The question which areas in the volume should
be masked as NaN. If we want to create a Pseudo CAPPI, only the areas beyond the range...

ENH: added plot_scan_strategy and plot_tseries to vis.py

plot_scan_strategy plots the beam at different elevation angles, plot_tseries
is a convenience function to plot time series

FIX: some edits to plot_plan_and_vert - this function is not mature, yet, so the...

FIX: made sure that axes in ErrorMetrics.plot begin at zero

Minor edits in util.py

FIX: fixed an error that occurs when writing and reading metadata in hdf5 files

ENH: added new adjustment classes GageOnly and AdjustNone

Both behave the same as the other adjustment classes, but the first only interpolates
gage observations, the second returns only the unadjusted values

FIX: fixed a bug that occurs for cross validation in two dimensions

FIX: Caught an error in polar_plot occurring with newer versions of matplotlib.
(v 1.1.0 and newer).
Due to changes in the API of Polar Transformations and two additions of
the standard PolarAxes object, our NorthPolarAxes does not work any more, but
it won't be needed any more either....

ENH: Added two functions to plot a horizontal plane view together with marginal vertical sections

The base function is vis.plot_plan_and_vert. vis.plot_max_plan_and_vert uses the
first function to plot the maximum values of a regular 3d grid along the 3 axes.