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

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.

MultipoleT now converges for curved magnets
MultipoleT is split into three derived classed and a base class

This reverts commit 69985bb1.