Add cell_statistics diagnostics

[zmaalick]

Implement first diagnostics feature: cell_statistics for precipitation.

[zmaalick]

• Implement cell effective radius
• Implement cell mean size
• Split cell statistics and plotting routines
• Add config file
• Move additional diagnostics file in subdirectory
• write unit tests
• validate figures with toolbox
• integrate in recipe/suite
• rewrite doc strings

[Sylviabohnenstengel]

first step requires updating the branch