Proposal for new chemical units

[saki7]

IU - International Units

This unit is specified for common chemicals where you can't determine effectiveness by its absolute value. In those cases IU is used, which varies among each chemicals.

For example, micrograms are commonly used in most of vitamin drugs, but sometimes there's exceptions (such as milligrams). IU is the standardized relative unit.

kind: :unitless (I'm not sure)
aliases: [IU, international-units]

Eq - Molar Equivalent

kind: :substance (I'm not sure)
aliases: [Eq, molar-equivalent]

We have to make sure that prefixed unit is available for this unit, e.g. mEq.

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Problem: You can't define a new unit which contains the % string.

I think this is related to the core parse routine, so I need your help on this.

v/v% - volume per volume

Specifies how much volume is contained in the substance.

kind: :unitless I guess. But if you are considering to create a new kind, :percentage, it might be more appropriate to use that one instead.
aliases: v/v%
definition: RubyUnits::Unit.new("1 mL") / RubyUnits::Unit.new("100 mL")

w/v% - weight per volume

Specifies how much weight is contained in the substance.

kind: same as v/v%.
definition: RubyUnits::Unit.new("1 g") / RubyUnits::Unit.new("100 mL")

[saki7]

Also, it would be very nice if you could add μ as an alias of micro prefix.

[saki7]

Related to #108.