Restructured the folders

[JulStraus]

The previous folder structure was a bit too annoying when working on the development. To this end, the following changes were done:

• All files creating types were moved to a folder called structures to avoid having a prefix structures_ in each file name.
• All tests start now with the name test_ to be consistent with the other packages.
• The file names in the test folder were updated to avoid ambiguities in naming.
• The example model was included in the file in which the tests are conducted.