add failing test for writing with file_sans_ext

* see bug #150
Append ext when file does not already have one

* closes #150
Failing test when file has no ext and explicit ext was set
fix case when file has no ext and no ext is set

* really closes #150
more examples of writing neurons
Merge branch 'fix/write.neuron_file_sans_ext'

* fix/write.neuron_file_sans_ext:
  more examples of writing neurons
  fix case when file has no ext and no ext is set
  Failing test when file has no ext and explicit ext was set
  Append ext when file does not already have one
  add failing test for writing with file_sans_ext
* closes #150
* prepares for #149