Add thermal parameters to worlds

[angelacmaio]

Adds the <atmosphere> tag with thermal properties for each world. In this PR, all worlds are set to:

<atmosphere type="adiabatic">
    <temperature>288</temperature>
    <temperature_gradient>0.2</temperature_gradient>
</atmosphere>

The gradient currently affects both temperature variation over altitude (K/m) and in-scene temperature variation. Soon, this should be broken out into separate parameters via a new plugin, so we can keep approximately this level of in-scene temperature variation and reduce the altitude-based variation.

@iche033 @nkoenig can you take a look?

[osrf-jenkins]

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