Improve online documentation

[bulli92]

Improve the Gallery and examples by putting them into ipynb

Use
a) examples in example directory
b) the following examples

Basic data analysis

Temporal mean

Calculating the temporal mean field of a variable is as simple as::

    air.timmean()

Spatial mean

The (area weighted) spatial mean is obtained as::

air.fldmean()

Masking an area

You probably want to work only on particular regions. The following script shows you how to easily to this.

.. plot:: ../../pycmbs/examples/05_mask_a_region.py
  :include-source:

Temporal slicing

If you want to perform a temporal subsetting of the data, this can be done as follows::

# temporal subsetting using existing start/stop dates
import datetime.datetime

start_date = datetime(2001,05,01)
stop_date = datetime(2010,04,15)
air.apply_temporal_subsetting(start_date, stop_date):

Working with multiple datasets

Simple arithmetic operations

.. plot:: ../../pycmbs/examples/03_data_analysis.py
  :include-source: