IfcCurveMeasureSelect non negative -> length measure

[SergejMuhic]

          I think that everything (with regards to sprint 2.1) for the 2.1 file should be ok now, except that the schema needs to be updated from 4.3 (i.e. IfcCurveMeasureSelect should use IfcLengthMeasure instead of IfcNonNegativeLengthMeasure).

Originally posted by @larswik in #198 (comment)

@larswik @Michel-Rives @pjanck @anborr was this not discussed in the reconciliation?