feat: ppm parts per million

[nebkat]

No description provided.

[JohelEGP]

For reference, the SI brochure mentions:

5.4.7 Stating quantity values being pure numbers
[...]
The term “ppm”, meaning 10⁻⁶ relative value, or 1 part in 10⁶, or parts per million, is also
used. This is analogous to the meaning of percent as parts per hundred. The terms “parts per
billion” and “parts per trillion” and their respective abbreviations “ppb” and “ppt”, are also
used, but their meanings are language dependent. For this reason the abbreviations ppb and
ppt should be avoided.
[In a side note:]
In English-speaking
countries, a billion is now
generally taken to be 10⁹
and a trillion
to be 10¹²; however,
a billion may still
sometimes be
interpreted as 10¹²
and a trillion as 10¹⁸.
The abbreviation ppt
is also sometimes read
as parts per thousand,
adding further confusion.

[mpusz]

Thanks!