Approximate factor for IfcConversionBasedUnit for mass

The documentation for [IfcConversionBasedUnit](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcConversionBasedUnit.htm) includes a list of conversion factors for commonly used conversion based units as Table 8.11.3.2.R.

In the table, 'pound' is described as 'Mass measure equal to 0.454 kg'. However, other mass measures such as 'ton US' include substantially more significant digits in the conversion factor.

The exact factor for converting pounds to kg is `4.535 923 7 E-01` per NIST Special Publication 811 Guide to the SI [Appendix B](https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors) and more specifically [footnote 22](https://www.nist.gov/pml/special-publication-811/nist-guide-si-footnotes#f22).

An unscientific sampling of IFC models using this unit define it in accordance with the NIST value, not the value in Table 8.11.3.2.R.