Add cell_statistics diagnostics

[zmaalick]

Implement first diagnostics feature: cell_statistics for precipitation.

[zmaalick]

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

[Sylviabohnenstengel]

first step requires updating the branch