Some performance improvements

[dholland88]

I have recently been performing some profiling while running CFD simulations with Zero-RK for finite rate chemistry. While doing this I found a few easy updates to make the code more efficient.

[whitesides1]

Thanks for the suggested changes. They both look like good improvements, but I don't want to accept them as is. Would you rather I make comments to improve the pull request or that I just make the changes myself?

[dholland88]

I'm fine with you making the modifications.

[whitesides1]

I decided to accept as is and then clean them up. This way your contributions are credited appropriately. Thanks for the input.