Implemented equals method for Quantity

[saskiawagenaar]

Solves #343.

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[saskiawagenaar]

Signed CLA now

[brendandburns]

LGTM, thanks!

[Hendrik-H]

Hi, when implementing equals one should also implement hashCode. For example like this:

@Override
public int hashCode() {
    return Objects.hash(number, format);
}