first implementation of bridging_sequence

* returns a list
make bridging_sequence return character vector
test equivalence of length 2 bridging sequences
don't simplify, this kills attributes
tests: bridging_sequence returns list
switch order of bridging_sequence to sample, ref

* for consistency with other functions
Add round trip test for via registration
some comments for bridging_sequence
Merge branch 'feature/bridging_seq'

* feature/bridging_seq:
  some comments for bridging_sequence
  Add round trip test for via registration
  switch order of bridging_sequence to sample, ref
  tests: bridging_sequence returns list
  don't simplify, this kills attributes
  test equivalence of length 2 bridging sequences
  make bridging_sequence return character vector
  first implementation of bridging_sequence