fix memory errors in unit tests as reported by valgrind: initialise array, check for container end, array deletion

reset output stream, fixes segfault in PeriodicBC when running multiple tests after each other (fix #570)

fix compile warnings: '‘bct’, ‘voffset‘, ‘coffset‘ may be used uninitialized in this function' and 'comparison between signed and unsigned integer expressions'

Merge branch '402-vffa' of gitlab.psi.ch:OPAL/src into origin/402-vffa - implement VFFA analytical field model.

 inbetween two elements. For this try to estimate length of longest
implicit drift. Then search for twice this distance for further
elements.