fix rounding in altium to kicad converter

The Altium -> Kicad importer introduces errors in the primitive dimensions that make it difficult for the user to work with the imported file without parsing the imported file through a cleanup phase. An example is 0.15mm tracks are changed to 0.149999mm tracks which make editing the imported file difficult. The reason for the error is that Altium Designer is based on imperial units, so rounding errors of up to 1.27nm occur, and the conversion to Kicad introduces up to 0.5nm of error. The imperial to metric conversion functions do not take this error into account.