fix bug in plotting mpsth objects of length 1

* had been broken for some time …
docs: links in plot.mpsth
remove CHECK notes by switching subset to [ calls

* subset using unquoted column names is idiomatic but upsets check
Remove another subset call to squash remaining CHECK note

* NB in this case note subset dropped NA expressions whereas the subset
  operator required an explicit is.na check to give the same behaviour
* only spotted this because of a test ...
Change split.spiketimes to match base split

* these arg names make little sense but we’ve
  already deprecated the function and the only uses
  I can find use positional not named arguments
add travis config
Add travis and zenodo shields
Merge branch 'feature/clean-check' into develop

* feature/clean-check:
  Add travis and zenodo shields
  add travis config
  Change split.spiketimes to match base split
  Remove another subset call to squash remaining CHECK note
  remove CHECK notes by switching subset to [ calls
  docs: links in plot.mpsth
  fix bug in plotting mpsth objects of length 1