[BUG] Bug in reading lammps trajectory files with random type id

[Angel-Jia]

dpdata assumes the types of elements in one file are the same, which is not true in some cases. For example, the trajectory file cannot be loaded correctly:

ITEM: TIMESTEP
0
ITEM: NUMBER OF ATOMS
3
ITEM: BOX BOUNDS xy xz yz pp pp pp
0.0000000000000000e+00 6.8043376809999998e+00 2.5385198599999999e-02
0.0000000000000000e+00 6.7821075796999999e+00 1.8630761460000000e-01
0.0000000000000000e+00 6.6801861338000004e+00 6.5204177000000002e-02
ITEM: ATOMS id type x y z
1 1 3.48873 0.0697213 6.67774
2 2 3.38621 0.033338 3.34239
3 3 1.79424 1.7281 5.01015
ITEM: TIMESTEP
10
ITEM: NUMBER OF ATOMS
3
ITEM: BOX BOUNDS xy xz yz pp pp pp
3.0951719137647604e-02 6.7713982144168243e+00 2.5146837349522749e-02
3.1535098850918430e-02 6.7499602284333751e+00 1.8455822840494820e-01
3.1362715442244227e-02 6.6488234183577575e+00 6.4591924584292706e-02
ITEM: ATOMS id type x y z
1 3 6.63593 3.49936 3.46086
2 2 3.44881 6.57204 3.4593
3 1 1.85117 5.11268 4.96295