Update gammapy.__init__ docstring
@@ -23,12 +23,12 @@ the following sub-packages (e.g. `gammapy.spectrum`):
`astro` --- Astrophysical source and population models
`background` --- Background estimation and modeling
`catalog` --- Source catalog tools
+ `cube` --- Cube analysis
`data` --- Data and observation handling
- `datasets` --- Access datasets
`detect` --- Source detection tools
`image` --- Image processing and analysis tools
`irf` --- Instrument response functions (IRFs)
- `morphology` --- Morphology and PSF methods
+ `maps` --- Sky map data structures
`spectrum` --- Spectrum estimation and modeling
`stats` --- Statistics tools
`time` --- Time handling and analysis
