documentation typo
export landmarks i/o functions

* and run examples (which would have caught this)
test for one ext when >1 registered

* encapsulates https://github.com/jefferis/nat/issues/194
Fix bug in setting ext when multiple registered

* closes https://github.com/jefferis/nat/issues/194
New baseline tests for getformatwriter

* encapsulates expectations of https://github.com/jefferis/nat/issues/195
* essentially no change when ext=NA
  and only add when ext=NULL
getformatwriter should not touch ext when ext=NA

* and should keep existing ext when ext=NULL
write.neuron should ignore extension from auto file name

* unless we have set ext=NA in which case we will try and use it
typo in getformatreader description
refix bug in setting ext when multiple registered

* closes https://github.com/jefferis/nat/issues/194 for good
Update getformatwriter docs with new behaviour
fix examples for read/write.landmarks