Resolve "Refactor unittest for core/structure module"

Summary of changes

Test cases have been added for the following functionalities:

• properties (positions, chemical-symbols, atomic-number, cell, unique-sites, pbc)
• setter and getter of below listed properties
• get position
• get distance
• find_lattice_site_by_position (simple, medium, hard) by Mattias
• structure from/to atoms
• repr function

Closes #132 (closed)