SK Rev. 4:
- Use of the 'Agg' backend if no display is available;
- New matplotlib rcParams to improve figure and plot styling;
- Added custom colormaps, especially for chlorophyll-a;
- New functions in common module --- natural_keys, latex_scientific, num2ymd;
- New class and funcitons in dynamics module --- constants; f; relation_dispersion_Rossby;
- Minor changes in file module;
- Minor changes in geostrophy module;
- Changes in gis, graphics and signal modules;
- New function in stats module: climatology;