move amira neuron io into separate source file
redesign ioregistry functions with writers in mind

* better separated functions for registering, querying and read
Add getformatwriter
exports/doc improvements
some basic test for neuronformats queries
(.)neuronformats -> fileformats - we will add e.g. 3d images soon
writer should return list for file format for consistency

* we might want e.g. the default file extension
documentation improvements/fixes
Merge branch 'feature/ioregistry2' (planning writers / image formats)

These changes will enable the addition of file writers and should generalise the
approach to other image types.

* feature/ioregistry2:
  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