Consider matching the API of other units software more closely

[matterhorn103]

Areas of the API to consider harmonizing more with other packages:

• Uncertainties are added in pint with plus_minus(), while in quanstants we have plusminus() (which is an alias for with_uncertainty()) - renamed alias to plus_minus() to match
• The equivalent of CompoundUnit in astropy is called CompositeUnit; maybe there is an example of the SI naming the product of unit multiplication?
• Parsing of units - thinking here mainly of GNU units, but pint would be another option to consider
• astropy has the DimensionlessUnit class, whereas quanstants has UnitlessUnit; they are different though, as a DimensionlessUnit can have a scaling factor and therefore is not necessarily equal to 1; quanstants.unitless is thus equivalent to astropy.units.unscaled_dimensionless
• astropy considers radians and steradians as base units and angles as a dimension, with an option to consider them dimensionless