Loss of precision when converting sdf files

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/125)) by Steve Peters (Bitbucket: [Steven Peters](https://bitbucket.org/%7B2ccfed09-18b8-4921-8d58-15ef01092802%7D/), GitHub: [scpeters](https://github.com/scpeters)).**

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We recently noticed some losses of precision when converting sdf files, and the cause may be that [ignition math stream operators have limited precision](https://bitbucket.org/ignitionrobotics/ign-math/issues/47/stream-operators-have-low-precision). This is not a problem when stream operators are only used for printing to the screen, but sdformat is using the stream operators for lexical casting, which causes much precision to be lost when converting sdf files.