WIP: Add somatostatin cell type

[wagdy88]

as title states

[jasmainak]

@wagdy88 I rebased the branch for you, so that the error should be fixed. Can you try to push forward now? To get this branch, do:

$ cd hnn-core
$ git checkout master
$ git branch -D somatostatin
$ git fetch origin somatostatin:somatostatin
$ git checkout somatostatin

Then work on test_script and make new commits and push them as you make progress. So we can help if you get stuck.