Update lower, upper, effort, and velocity default joint limits

[Amronos]

Changes from ros/urdfdom#212 (comment) and #83 (comment).
ros/urdfdom#212 should be updated accordingly, or a new PR should be created to update the defaults of these limits.

[Amronos]

@saikishor @scpeters please take a look whenever you get the time.

[saikishor]

LGTM

[scpeters]

Thanks for opening the PR!

One thing we need to guarantee is that the behavior of existing URDF (most importantly version 1.0 but also 1.1) does not change. To ensure this, I would need to see a corresponding pull request to urdfdom that does the following:

• updates the default values of these fields in the URDF schema (see urdfdom's xsd/urdf.xsd:211-214)
• updates the version documentation in the README
• adds tests confirming that behavior does not change when parsing previous URDF versions

this may be a little more complicated to implement than I thought, but this is a very widely used file format, and we need to preserve existing behavior.

[Amronos]

I don't have the time right now to make a proper PR to urdfdom due to exams but would still like to see this included in the version 1.2 of the spec.
@saikishor Could you please update ros/urdfdom#212 or make a new PR and handle this for me?
If that's not possible, a PR from me would probably come during early April.

[saikishor]

@Amronos @scpeters I can add the changes in a different PR once ros/urdfdom#212 is merged. I would prefer to leave them separate for now.

Is that fine for you?