ENH: Module 'connection_m' was cleaned up, so the dummy Neuron class was moved to the separate module 'neuron_dummy_m'.

FIX: Added script 'run_tests.sh for running all tests together and implemented this into .gitlab-ci.yml

ENH: Added Makefiles and Connection module separated into two file - one with module itself and the other one for testing purposes.

ENH + FIX: Added comment to multiple methods + fixed constructor implementation for the class IntervalConnection.

    - New documentation can be generated with 'doxygen' command in
      the root folder
    - Documentation will be automatically saved in the folder 'docs'

    contains:
        - module Connection_mod with classes Connection and
          IntervalConnection

    TODO:
        - delete dummy Neuron class after its implementation
        - fully comment the code