v6.1.5

changelog:

• separated devel package
• stddev subtype in stencilinter type
• prefer seconds for timedelta to avoid problems with long timeranges
• timedelta can be initialised in sec as well
• fix bitmap key in grib2
• handle better missing values in vertical levels
• correct order for B14018
• handle correctly stdin and stdout in dballe_class
• implemented n-1 variant of variance and stddev, variance guaranteed >=0