Add static_asserts to RAJA::kernel

[davidbeckingsale]

[willkill07]

I'd just like to comment that I have complete static_assert machinery in the code for #237. Extending checking like this for forallN shouldn't be too difficult. Do you have any idea on the requirements that you'd like to ensure for less-awful compiler errors?

[davidbeckingsale]

What you have in #237 looks good. I don't think there were any specific requirements here, just the notion of doing what we can to insulate a user from a screenful of errors.

[willkill07]

At this point each invocation to forall is checked accordingly. We also have checks already in place mandating that a user has the same number of segments as they do execution policies. I'm not sure how much more we can / should be checking for with the work being done with nested::forall

[trws]

Rather than offing this, I'm converting it to an action item for the new interface, we could definitely use some better error messages in there.