Add scale parameter to blackbody emitter [feature]

[alhirzel]

Description

This PR adds the scale parameter back to the blackbody emitter. This feature is ported from the old Mitsuba and is useful when scenes are not expressed in meters, or when simulating a gray body.

Testing

Please describe the tests that you added to verify your changes.

Checklist:

Please make sure to complete this checklist before requesting a review.

• [x] My code follows the style guidelines of this project
• [x] My changes generate no new warnings
• [ ] My code also compiles for gpu_* and packet_* variants. If you can't test this, please leave below
• [x] I have commented my code
• [x] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I cleaned the commit history and removed any "Merge" commits
• [x] I give permission that the Mitsuba 2 project may redistribute my contributions under the terms of its license

[alhirzel]

I removed WIP but haven't run the tests formally yet. What level of test would be expected for this change?