Add decay length significance `BPVDLS` function in Thor
The LoKi variable BPVDLS
is not available in Moore
now. The definition of this decay length significance variable is in https://lhcb-doxygen.web.cern.ch/lhcb-doxygen/lhcb/v50r2/db/d89/_particles37_8cpp_source.html#l00078 . We propose using information available in MR !2486 (closed) to implement the variable in Thor.