Allow custom formatting of magnitude of a quantity

[maekl]

It would be nice to be able to customise the formatting of the magnitude of a quantity.

Maybe by introducing ShowMagnitude similar to ShowUnit and ShowDimension.

trait ShowMagnitude[A] {
  def apply(a: A): String
}

Default instance could use toString or maybe even it makes sense to use DecimalFormat (with a couple of significant digits) as the default for Double and BigDecimal

[zainab-ali]

Thanks for the suggestion @maekl !

Yes, I think we could benefit a lot from this typeclass.

The current Show implemenation isn't particularly principled. We're using an implicit toString in q.value:

    implicit def quantityShow[A, D <: HList](
        implicit showUnit: dimensions.ShowUnit[D],
        showDimension: dimensions.ShowDimension[D]
    ): Show[Quantity[A, D]] =
      new Show[Quantity[A, D]] {
        def apply(q: Quantity[A, D]): String =
          s"${q.value} ${showUnit().trim} [${showDimension().trim}]"
      }

It would be better if this used a show instance for magnitude instead.
Is this something you'd be willing to submit a PR for?