Area definition as part of static dataset files

[joeloskarsson]

In order to make the code reusable with different limited area configurations, the exact definition of the model area should not be hard-coded in constants.py.

Proposition

Let a file area_def.json, stored in data/my_dataset/static describe the model area. This description should include:

• The projection used for defining the area (useful for plotting).
• The extent of the area in the coordinate system of the projection.
• The grid shape (N x M).

These things are currently contained in constants.py. The json file could then be used to create an AreaDefinition object that could be passed to the model, plotting scripts etc.

[joeloskarsson]

Superseded by #23