v0.3.0 - descriptor arithmetic

new features

• issue doi using zenodo (fe8e2fe)
• descriptor arithmetic (7e10cb4)
• descriptor name dictionary (59a0e82)

broken changes

• raise error when Descriptor.__call__ errored (906d160)
• change all_descriptors generator to descriptors module (8649174)

bug fix

• Descriptor.__repr__ (0f2b1b9)