v1.21.0

Unitful v1.21.0

Diff since v1.20.0

Merged pull requests:

• support date/time + uniful arithmetic (#731) (@aplavin)
• Release v1.21.0 (#733) (@sostock)

Closed issues:

• AMU and g/mol are not dimensionally compatible?? (#651)
• How to cite Unitful.jl? (#725)
• round(x::Quantity) = round(typeof(x), x) (#732)