Clean up

• updated license
• updated build-backend to hatchling
• fixed sphinx issues
• reader outputs masked arrays if needed now
• reader was optimized in performance (reading recreated info/grid for each update call)
• minor internal refactoring
• made examples work again
• added writing of variable attributes from Info.meta
• fixed reading of axes attributes to Grid.axes_attributes
• reworked Layer mechanics in !38 (merged)