test callbonds functionality thorougly
The callbonds function needs to allow for quite a number of combinations of data types, since both, couplings and statevectors can be scalar- or vector-valued. This boils down to having the necessary overloading of the mult
function available and using them correctly. Right now most of those are not properly tested!
Note: Major changes in #a6335a35 where I also added a few remarks as comments