Makes a new name for an existing unit.

This makes a new name for an existing unit, for example you might want $ as a short name for USD.The key difference between using an alias and creating a new unit is that if you want to swap between two separate units, you must use a conversion function, whereas with an alias you do not. So if you have two units that are just different names for the same unit (for example, a shorter abbreviation for inch), use an alias. If you have two different units that happen to be equivalent (for example, you might want a unit bagOfSugar which equals 1 kilogram, but they are not measuring the same thing), define a full unit instead.