register rds format with reader and writer
(failing test) for getformatwriter
fix return value and handling of ext in getformatwriter
defensively trim file leading period from extension in fileformats and registerformat

* also more docs and tests
allow partial argument matching of formats
more sophisticated handling of file extensions (which now include period)
Merge branch 'feature/ioregistry2'

These changes will enable the addition of file writers and should generalise the
approach to other image types. They also include logic to return an appropriate
filename to a writer.

* feature/ioregistry2:
  more sophisticated handling of file extensions (which now include period)
  allow partial argument matching of formats
  defensively trim file leading period from extension in fileformats and registerformat
  fix return value and handling of ext in getformatwriter
  (failing test) for getformatwriter
  register rds format with reader and writer
  documentation improvements/fixes
  writer should return list for file format for consistency
  (.)neuronformats -> fileformats - we will add e.g. 3d images soon
  some basic test for neuronformats queries
  exports/doc improvements
  Add getformatwriter
  redesign ioregistry functions with writers in mind