Add newton_per_square_millimeter in Pressure about uom

[iliekturtles]

Easily!

Add a definition for the unit in the units { ... section:

uom/src/si/pressure.rs

Lines 36 to 40 in 34538a9

	@zeptopascal: prefix!(zepto); "zPa", "zeptopascal", "zeptopascals";
	@yoctopascal: prefix!(yocto); "yPa", "yoctopascal", "yoctopascals";
	@atmosphere: 1.013_25_E5; "atm", "atmosphere", "atmospheres";
	@atmosphere_technical: 9.806_65_E4; "at", "atmosphere (technical)",

And a test at the bottom:

uom/src/si/pressure.rs

Lines 131 to 135 in 34538a9

	test::<f::zeptonewton, a::square_meter, p::zeptopascal>();
	test::<f::yoctonewton, a::square_meter, p::yoctopascal>();
	test::<f::dyne, a::square_centimeter, p::dyne_per_square_centimeter>();
	test::<f::kilogram_force, a::square_millimeter, p::kilogram_force_per_square_millimeter>();

PRs welcome, otherwise I can get to this after reviewing a couple open PRs.

[Eagle941]

Thank you for your answer! I have just opened #397