DOC: Removed from Gettig Started installation instructions everything that refers to the BUFR software.

This is because BUFR software is now an optional dependency, so users shoudl not be
bothered with it during regular installation.

DOC: Minor typo.

DOC: Referred to the doc section about optional imports in the exception raised by class OptionalModuleStub.

DOC: Added a doc sub-section "Optional Dependencies" to the "Getting Started" section.

Please beware that the content of this section is preliminary. We need to add
more info o how to install the BUFR module. In tzhe future, we need to take care of keeping...

DOC: introduced a more telling default mode for correctAttenuationHB.

Changed the docstring accordingly

FIX: fixed a bug in correctAttenuationHB.

- the checks for attenuation overflow looked at the wrong gate
- the 'nan' and 'zero' mode assigned their values to the wrong gate

DOC: Clarified the new behaviour of read_RADOLAN_composite in the function docstring.

ENH: refined optional_import to allow global imports.

function now always returns an object (the actual module on success,
a stub object on failure), so that imports may also be done globally
without breaking the code on a failed import. The lazy import will be...

MIN: Some minor edits in the use of the deprecated decorator.

DOC: Removed reference to speedup from the module docstring.

Some other minor edits in the module docstring.

MIN: Marked several functions as depercated in the dp module.

The functions are fill_phidp and process_raw_phidp.

MIN: Marked several functions as deprecated in the vis module using the new decorator.

Decorated as deprecated are:

PolarPlot --> replaced by plot_ppi
polar_plot2 --> replaced by plot_ppi
polar_plot --> replaced by plot_ppi
CartesianPlot --> no replacement...

ENH: Added a deprecation decorator function in the util module.

The decorator can be used in order to mark deprecated functions. If these functions
are called, a DeprecationWarning will be raised.

In order to use the decorator, just

>>> from util import deprecated...

MIN: Release version is now specified in text file version, not in setup.py anymore.

REF: Removed anything related to the BUFR software from wradlib setup.

Now, wradlib should install as a pure python package.

MIN: In the global section of module bufr, we now check whether the BUFR software is operational.

If it turns out that the BUFR software (mainly decbufr) is not operational on
that system, module bufr will raise an ImportError. In that case, util.import_optional...

MIN: The import of the bufr module is now via lazy loading in read_BUFR in module io.

The global import of the bufr module in the io module was removed.

FIX: Fixed an error in the call of import_optional

MIN: removed import of module bufr from module __init__.

MIN: Using lazy loading and optional import for the speedup module in module dp.

This means the dp does not try anymore to globally import speedup. Insstead, speedup
is imported by util.import_optional within the function dp.unfold_phi.

No change in behaviour, execpt no warning will show if you run dp but not use...