now always throws error if a bond type changes, but both of of the involved atoms are too close to edge of template

- improve error detection and messages
- avoid sigma/cutoff to be zero (and thus division by zero)
- move new/delete of temporary storage outside of loop

Added pair styles lj/expand/coul/long to USER-MISC and its gpu version to GPU package, added lj/cut/dipole/long/gpu; and added an example Makfile to build GPU package for multi-arch

- in fix rigid, rigid/small, and poems, a string argument was
  incorrectly assigned to a %d format
- plug two small memory leaks in fix bond/react

provide keyword/value option to compute ackland/atom for selecting legacy or current variant of implementation