Pandas assignments

```
import pandas as pd
from msmbuilder import assignments, io, MSMLib

ass_msmb2 = io.loadh("./Ward-3120/Assignments.Fixed.h5")["Data"]
ass = assignments.convert_array_to_pandas(ass_msmb2)

C = assignments.get_count_matrix_from_assignments(ass)
C0 = MSMLib.get_count_matrix_from_assignments(ass_msmb2)

C - C0

C = assignments.get_count_matrix_from_assignments(ass, lag_time=5)
C0 = MSMLib.get_count_matrix_from_assignments(ass_msmb2, lag_time=5)

C - C0

C = assignments.get_count_matrix_from_assignments(ass, lag_time=5, sliding_window=False)
C0 = MSMLib.get_count_matrix_from_assignments(ass_msmb2, lag_time=5, sliding_window=False)

C - C0
```