BUG : Fix pandas import error in Group_landmark.py file when loading AQ3DC

[HUTIN1]

The importation of pandas in the Group_landmark.py file makes the loading of AQ3DC crash, if the user doesn't already have pandas in Python slicer. And AQ3DC was not able to install pandas, because the importation of Group_landmark was before the installation of pandas.

@allemangD Thanks in advance for reviewing the pull request

[HUTIN1]

For the commit STYLE : add comment in Measure file, rename variable and function

• Some variables containing a dictionary are renamed by writing dict in the name.
• Add comments in the Measure, Angle and Distance class to explain what can contain the classes. Particularly for Angle the meaning of its variable.
• Remove the "position" setting in the setitem function to create the setPosition function, because it was not Pythonic the way it was implemented.
• One or two functions were renamed to have a better understanding of their functionality.