Color tags could accept other formats

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/195)) by Louise Poubel (Bitbucket: [chapulina](https://bitbucket.org/%7B5cfa2075-477b-4ded-bdb9-8d2479544ec4%7D/), GitHub: [chapulina](https://github.com/chapulina)).**

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SDF color tags currently accept normalized color values where each color component can be a value between 0 and 1.

It would be convenient if SDF also understood other formats, such as HEX and 0-255 ranges.

For example, all the following would describe pure red:

~~~
1.0 0.0 0.0 1.0
255 0 0 255
#ff0000ff
~~~