Define sdf using XML Schema instead of custom schema

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/138)) by Shane Loretz (Bitbucket: [Shane Loretz](https://bitbucket.org/%7B656e3311-aad9-45a1-aaf7-b0ee0e84b287%7D/), GitHub: [sloretz](https://github.com/sloretz)).**

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I'm a little confused on how the SDF is defined, so this issue is partly to help me understand.

It appears SDF is defined by a bunch of ".sdf" files. The content of these files appears to be custom, not used outside of the sdformat project, but having the same purpose as [XML Schema](https://www.w3.org/XML/Schema). `sdf` files are validated against these ".sdf" files using a custom parser.

There does seem to be a demand to use XML Schema indicated in #2 and [pull request #91](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/pull-requests/91). The result was a script that translated from the custom format to XML schema.

It seems that the ".sdf" files could be replaced by XML Schema, and the parser could be replaced with another [open source library](http://xerces.apache.org/xerces-c/).

* https://www.w3.org/XML/Schema
* http://xerces.apache.org/xerces-c/
* https://cmake.org/cmake/help/v3.1/module/FindXercesC.html

Start by reviewing the custom .sdf schema files, the custom parser, and the translation script described in the issue. Compare the current approach with XML Schema and Xerces-C; done would require an agreed migration plan and scope, which the issue does not yet provide.

Written by the indexing model from the issue text.