Repetitions for grain boundary creation

[aazocar]

When creating a grain boundary the repetition is not functional. For example:

System.create.defect.grain_boundary(axis=[1,-1,0], sigma=3, gb_plane=[1,1,-2],
                    element = 'Fe',
                     structure = 'bcc',
                     lattice_constant = 2.83441,
                     uc_a=2, uc_b=2,
                     repetitions=(1, 1, 2),
                     graph = kg
                     )

[srmnitc]

Leave gb_code input as is, aimsgb input as in. Will have less functionality. Mode inbuilt should combine both, and have args which are common. Combine both features if possible.

[srmnitc]

In #163, the functions have been modified to support all arguments of GBCode and AIMSGB as they are. Docstrings are also updated to explain what arguments are used with each, and what the args mean. We still need to implemented a 'combined' method.