allow coefficient for additive coag kernel to be set from spec file

[zdaq12]

Resolves #186

[jcurtis2]

While the code looks good and accomplishes the purpose, I am wondering if env_state_t is the place for this variable beta_1 to reside. While it was least disruptive to existing code to place there as the coagulation kernel functions already take the env_state_t, my possible alternative suggestion would be placing it in scenario_t (and then passing that structure around). I think one could argue that this scaling factor is pretty similar in purpose to the wall loss parameters used for chamber_t which is a member of scenario_t.

@mwest1066 any thoughts?

[mwest1066]

Are we thinking of these parameters as being something point-wise in space that affects the coagulation kernel? E.g., something like a turbulent kinetic energy that might affect a coagulation kernel? If so, then I think that this is something which could vary in space and time, and so it does make sense to think of it as similar to a temperature or pressure, and hence to be included in env_t.

[mwest1066]

I don't like the name of this variable (it's very mysterious until you read the comment). Could we call it something like additive_kernel_coefficient instead?

[jcurtis2]

Changed from beta_1 to additive_kernel_coefficient throughout code in 5177a46