Cube source providers should generate reports

[forman]

This is an idea: each time we update the cube with a cube source provider, it should generate a report which can be used as input to the product handbook. The report includes

• a general description of the provider
• the source files that have been processed, the source's metadata and its provenance
• the transformations applied to the source data to fit it into the data cube's grid
• maybe also some basic statistics of the data processed

Such reports could be directly included into the product handbook if they are formatted using RST markup.

[meggart]

I very much like this report idea.

[forman]

See #5