Allow parsing SDF from `std::istream`

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/172)) 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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Currently the API for parsing SDF has two methods `readFile` and `readString`. Reading from an `std::istream` would give users more options in where the SDF content comes from.

The implementations of `readFile` and `readString` could be replaced with a call to the `std::istream` version. `readFile` would pass in an open `std:ifstream` while the `readString` would pass in `std::stringstream`.

Locate the SDF parsing API implementations for readFile and readString, then trace how their input reaches the parser. Add an std::istream-based parsing entry point while preserving the existing methods through stream inputs, and verify that file, string, and stream sources produce the expected SDF result.

Written by the indexing model from the issue text.