Resolve "Refactor unittest for core/structure_container class"

Description

Changes in TestsStructureContainer and TestFitStructure classes were made to increase coverage for structure_container.py (96%)

Comments

During the development of this work a bug was found and reported in issue #135 (closed)

Summary of changes

• Arguments required to initialize class are collected in __init__ before setUp.
• Test cases are added to cover methods related to repr function.
• Test cases are overload to cover more code, i.e., testing sc.get_structure() and sc.get_structure([0]) in the same TestCase.

Closes #133 (closed)