Assert that box sizes are not zero.

[proteneer]

We should assert that for rectangular boxes, the diagonal elements are zero, as a box size of zero is just undefined behavior.

jfass pointed out that it probably suffices to just do this check in the context.

[maxentile]

Perhaps the context constructor should even assert something stronger, e.g. that the minimum box edge length >= cutoff / 2?

[badisa]

We now check that the box edge lenght >= cutoff / 2. This check is in the non-bonded potential, though verifying it in the context would be good as well.

[maxentile]

Checking in the context constructor made more sense when the box dimensions were constant for the lifetime of the context -- seems better for this check to be in the nonbonded potential now...

[badisa]

Can we close this?