Rename DecimalFraction units to Decimal

[angularsen]

Decimal fraction is not a correct term.

Definition of Decimal Fraction
A fraction where the denominator (the bottom number) is a power of ten (such as 10, 100, 1000, etc).
Source.

So, to convert a fraction to a decimal, divide the numerator by the denominator.
Source.

Changes

• Add Ratio.Decimal unit, mark DecimalFraction unit as obsolete
• Add RatioChangeRate.DecimalPerSecond, mark DecimalFractionPerSecond unit as obsolete
• Change base unit of each quantity to the new unit

Changing the base unit should not be a breaking change for most users and the conversion precision is exactly the same as before for all units.

[angularsen]

Related to #879

[lipchev]

I'm not sure about this- the linked definition emphasizes the distinction between 1/2 (fraction) and 5/10 (decimal fraction). However what we're representing is the right hand side of the equation: e.g. 0.53 which is the decimalvalue representation of a fraction (i.e. using a decimal symbol).

[angularsen]

@lipchev Good point, some more googling and I believe decimal is the correct term?

So, to convert a fraction to a decimal, divide the numerator by the denominator.
https://www.bbc.co.uk/bitesize/topics/znmtsbk/articles/z4ymtv4

[angularsen]

Updated PR to use Decimal and updated both Ratio and RatioChangeRate.

[MarkCiliaVincenti]

The problem with Ratio.Decimal is that decimal is a type in C#, so it could create confusion.

[angularsen]

@MarkCiliaVincenti Right, but since its type is double I think maybe not a big problem.
After reading up on the terms I believe DecimalFraction is not a good choice either.

Are we happy with Decimal? Any other suggestions?
@lipchev@MarkCiliaVincenti

[lipchev]

I personally prefer DecimalFraction over Decimal: we're represents a Ratio in terms of a Decimal Fraction expressed (visually) using the

modern European decimal notation

I also think that renaming this would have quite a visible effect on client code (having obsolete warnings popping all over the place), that it simply wouldn't be worth the effort..

[angularsen]

@lipchev Ok, I had not considered the "visual representation" part. I guess it makes sense when reading the wiki section.

https://en.wikipedia.org/wiki/Decimal#Decimal_fractions

The numbers that are represented by decimal numerals are the decimal fractions (sometimes called decimal numbers),
For example, the numerals

represent the fractions

I guess we could call it DecimalNumeral, but that is a very unfamiliar term to me at least.
I agree, not worth the hassle. Let's stick with DecimalFraction.