DictionaryTreeBrowser.add_dictionary was deleting the content of nodes that already in the tree.
Add test for previous bug.
Automatic style corrections courtesy of autopep8.
Merge pull request #225 from francisco-dlp/BUG_FIX_DictionaryTreeBrowser

Bug fix dictionary tree browser
Merge branch 'master' of https://github.com/hyperspy/hyperspy into parafit
fixed peak shape
Automatic style corrections courtesy of autopep8