Add doxygen comments to dust code

[helenarichie]

Updated existing comments in dust_cuda.cu and added Doxygen comments to dust_cuda.h, dust_cuda.cu, and dust_cuda_tests.cpp. I'm unsure I used \param[in] and \param[out] appropriately in the function doc strings dust_cuda.h. I think that \param[out] should be used for an input parameter that I'm modifying, but not returning in the function, but in the case of dev_conserved I don't know if I should specify the field that I'm modifying or something like that. If anyone has thoughts on this, please let me know!

[bcaddy]

Here's what the \param in/out stuff means

• \param[in] is for variables that are input only and will not be modified in the function. They should also be declared as const or pass by value in the function definition to enforce no unexpected changes
• \param[out] is for variables that are output only. Their initial values will not be used and they should be passed in by reference or they should be a pointer type. Generally in this case you should just be returning the value from the function with a return statement; possibly with structured binding if needed.
• \param[in, out] is for variables that are both input and output. Their initial values will be used and the will also contain output; they should be passed in by reference or they should be a pointer type. Generally think about if this can be replaced with a pure input variable and a return statement.

dev_conserved is almost always either [in] or [in,out].

[evaneschneider]

It looks like this failed on one of Bob's new MHD tests - can you try updating the submodule again?

[evaneschneider]

(It's probably my fault for auto-merging in the most recent PR - I guess that doesn't update the submodule.)