Statement on units made more visible

(suggested by candrewn@gmail.com)
More robust checks with replica exchange and neighbor list

Now the code crashes in the following cases:
1. An exchange is attempted on a step where neighbor list for COORDINATION
(or DHENERGY) are not updated
2. An exchange is attempted and neighbor list for PATHMSD or PROPERTYMAP are enabled.

The second should be fixed better to allow using strides for lists which are
divisor of the stride between steps. The better thing will likely come later, at least
the code is safe now.