Change throat.direct_length calculation to use float64

[TKlokgieters]

Previously it was int64 which gave accuracy issues (especially for small voxel sizes);

For example with a voxel size of 1e-4 and similarly sized pores and throats; the direct_length calculation outputted zeros.
Now direct_length corresponds with openPNM models.network.pore_to_pore_distance