Add Syntool preprocessing capabilities

[aperrin66]

Add the ability to convert downloaded dataset files to the format needed to display it in Syntool. Also added the possibility to trigger harvesting via a task. At the same time, I altered a bit the structure to split processing tasks with different themes/scopes in different modules:

• tasks.core: operations that can be applied to all datasets like download or archive.
• tasks.idf: IDF conversion
• tasks.syntool: Syntool conversion
• tasks.harvesting: harvesting

73 was also merged in here, adding the ability to crop dataset files to a given bounding box.

[aperrin66]

Discussed with @akorosov , merging.