add examples structures for testing

tests/data/mp-149_Si2.cif

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+data_image0
+_chemical_formula_structural       Si2
+_chemical_formula_sum              "Si2"
+_cell_length_a       3.8492784033699095
+_cell_length_b       3.8492794116013456
+_cell_length_c       3.849278
+_cell_angle_alpha    60.00001213094421
+_cell_angle_beta     60.00000346645984
+_cell_angle_gamma    60.00001097545789
+
+_space_group_name_H-M_alt    "P 1"
+_space_group_IT_number       1
+
+loop_
+  _space_group_symop_operation_xyz
+  'x, y, z'
+
+loop_
+  _atom_site_type_symbol
+  _atom_site_label
+  _atom_site_symmetry_multiplicity
+  _atom_site_fract_x
+  _atom_site_fract_y
+  _atom_site_fract_z
+  _atom_site_occupancy
+  Si  Si1       1.0  0.875  0.8749999999999999  0.8750000000000001  1.0000
+  Si  Si2       1.0  0.12500000000000003  0.125  0.12499999999999997  1.0000

tests/data/mp-2998_BaTiO3.cif

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+data_image0
+_chemical_formula_structural       Ba2Ti2O6
+_chemical_formula_sum              "Ba2 Ti2 O6"
+_cell_length_a       5.666408035279349
+_cell_length_b       5.6664076443956235
+_cell_length_c       5.668373884507181
+_cell_angle_alpha    119.98853562968743
+_cell_angle_beta     119.98853684518572
+_cell_angle_gamma    90.00000405170597
+
+_space_group_name_H-M_alt    "P 1"
+_space_group_IT_number       1
+
+loop_
+  _space_group_symop_operation_xyz
+  'x, y, z'
+
+loop_
+  _atom_site_type_symbol
+  _atom_site_label
+  _atom_site_symmetry_multiplicity
+  _atom_site_fract_x
+  _atom_site_fract_y
+  _atom_site_fract_z
+  _atom_site_occupancy
+  Ba  Ba1       1.0  0.7499999999999999  0.24999999999999997  0.5  1.0000
+  Ba  Ba2       1.0  0.24999999999999994  0.7499999999999999  0.49999999999999994  1.0000
+  Ti  Ti1       1.0  0.5  0.49999999999999994  0.0  1.0000
+  Ti  Ti2       1.0  0.0  0.0  0.0  1.0000
+  O   O1        1.0  0.75046047  0.25046046999999994  0.0  1.0000
+  O   O2        1.0  0.7495395300000001  0.7504604699999999  0.0  1.0000
+  O   O3        1.0  0.25046047  0.24953952999999995  0.0  1.0000
+  O   O4        1.0  0.24953952999999995  0.74953953  0.9999999999999999  1.0000
+  O   O5        1.0  0.25  0.24999999999999997  0.5  1.0000
+  O   O6        1.0  0.75  0.7499999999999999  0.5  1.0000

tests/data/mp-66_C2.cif

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+data_image0
+_chemical_formula_structural       C2
+_chemical_formula_sum              "C2"
+_cell_length_a       2.5178271667719256
+_cell_length_b       2.5178269134954783
+_cell_length_c       2.51782692
+_cell_angle_alpha    59.99999991454223
+_cell_angle_beta     60.00000324214051
+_cell_angle_gamma    59.9999988338285
+
+_space_group_name_H-M_alt    "P 1"
+_space_group_IT_number       1
+
+loop_
+  _space_group_symop_operation_xyz
+  'x, y, z'
+
+loop_
+  _atom_site_type_symbol
+  _atom_site_label
+  _atom_site_symmetry_multiplicity
+  _atom_site_fract_x
+  _atom_site_fract_y
+  _atom_site_fract_z
+  _atom_site_occupancy
+  C   C1        1.0  0.8750000000000001  0.875  0.8749999999999998  1.0000
+  C   C2        1.0  0.125  0.125  0.125  1.0000