Refactor packaging

Hi all,

This MR aims to improve the packaging of the toolbox. In general, deleting globals tend to have unwanted consequences. For instance, I have encountered import problems during unit testing. The refactoring has a similar outcome as the previous implementation, which is clean namespace. This is due to the __all__ being listed to each modules, as suggested here.

I also have listed all the required libraries for the Toolbox. One can now freely install the library on a local environment with pip install -e ".[maxwell]". The maxwell tag is a side effect of a direction that I would like to suggest. That is, the Toolbox codes can be used elsewhere; maybe in conjunction with another library, maybe as a Karabo device.

Please feel free to let me know your thoughts :)

@lleguy @mercurio @mercadil @teichman

doc/getting_started.rst

@@ -15,10 +15,10 @@ If the toolbox has been installed in your home directory previously, everything
 
 .. code:: bash
 
-    pip install --user .
+    pip install --user ".[maxwell]"
 
 Alternatively, use the -e flag for installation to install the package in development mode.
 
 .. code:: bash
 
-    pip install --user -e .
+    pip install --user -e ".[maxwell]"