Allow blackbody param type

[wahn]

The updated/new scene from the PBRT input file documentation contains a blackbody parameter:

http://www.pbrt.org/fileformat-v3.html#example

> rg blackbody assets/scenes/example.pbrt -B 1
17-LightSource "distant"  "point from" [ -30 40  100 ]
18:   "blackbody L" [3000 1.5]

Currently the parser can't handle it:

> ./target/release/examples/pest_test -i assets/scenes/example.pbrt 
FILE = assets/scenes/example.pbrt
search_directory is /home/jan/git/self_hosted/Rust/pbrt/assets/scenes
992 bytes read
thread 'main' panicked at '  --> 18:4
   |
18 |    "blackbody L" [3000 1.5]
   |    ^---
   |
   = expected statement, parameter, or last_statement', examples/pest_test.rs:1400:44
note: Run with `RUST_BACKTRACE=1` for a backtrace.

The task is to allow those parameters and to handle them before calling e.g. the constructor of a light.

[wahn]

Commit 73b7666 resolves the issue.

The test scene renders fine now:

> imf_diff -d -f simple.png pbrt.png diff.jpg
differing pixels:	  0.338% (541 of 160000)
average difference:	  2.137%
maximum difference:	  7.692%
Summary: Some pixels differ slightly.
== "simple.png" and "pbrt.png" are similar

There is a difference to the C++ version, but that's not too bad: