BF: Fix Cython signed vs. unsigned integer comparison warning
BF: Fix Cython signed vs. unsigned integer comparison warning

Fix Cython signed vs. unsigned integer comparison warning.

Fixes:
```
dipy/tracking/distances.c: In function
â€˜__pyx_f_4dipy_8tracking_9distances_min_distancesâ€™:

dipy/tracking/distances.c:8874:41: warning:
comparison between signed and unsigned integer expressions [-Wsign-compare]

for (__pyx_v_t2_pi = 0; __pyx_v_t2_pi <
__pyx_t_1; __pyx_v_t2_pi++) {
                                         ^
```

and
```
dipy/tracking/distances.c:11524:38: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
__pyx_t_12 = ((__pyx_v_mid_index < (__pyx_v_t_len - 1)) != 0);
                                 ^
```

raised for example in:
https://dev.azure.com/dipy/dipy/_build/results?buildId=373&view=logs&j=ad7c9750-8a16-5492-49ac-3815bac6d5d2&t=baa9a672-c8e1-55fc-55ff-b7fd664eb7f3&l=652
and
https://dev.azure.com/dipy/dipy/_build/results?buildId=373&view=logs&j=ad7c9750-8a16-5492-49ac-3815bac6d5d2&t=baa9a672-c8e1-55fc-55ff-b7fd664eb7f3&l=671

FrancoisRheaultUS/dipy,FrancoisRheaultUS/dipy