v0.5.1 🌌

Changes

• Major bug fixes and SpectralAsimovDataset @StFroese (#30)
• update notebook @StFroese (#28)
• Correct units for ULs @StFroese (#26)
• Rename save methods to write @StFroese (#24)
• Fix astropy version since gammapy uses deprecated function @StFroese (#27)