Not yet fixed for mixtures

Fix reducing density for nitrogen; see https://github.com/CoolProp/CoolProp/commit/fe7f6db631f373d1c49948b0ca26b8592b3bbf15

This reverts commit e6ef9f22.

The additional digits are correct according to the reducing density of 322 kg/m3 and the molar mass of 18.015268.  The value in REFPROP 10.0 is rounded more than necessary, and has been fixed.

No idea where the additional 0906 came from

See https://pubs.acs.org/doi/abs/10.1021/acs.iecr.1c00847