call init_stroh() only if we need it.
call init_stroh() only if we need it.

Allows to create an instance of the class without importing atomman. test_gamma_line() is an example of such scenario when we just want to access the data stored in the dislocation class such as unit_cell.

libAtoms/matscipy,libAtoms/matscipy,libAtoms/matscipy,libAtoms/matscipy