make implicit_spring_damper default to true

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/127)) by John Hsu (Bitbucket: [hsu](https://bitbucket.org/%7B0a186eae-abf0-4514-a951-23db5eccc286%7D/), GitHub: [hsu](https://github.com/hsu)).**

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There are too many simulation problems that are caused by explicit damping. I find myself telling people repeatedly to do:

~~~
add following blocks to your urdf for every joint:

1

~~~

or

~~~
add following block to your joint sdf:

1


~~~

I can't remember a time when explicit damping is preferred.