Fix tt min

It turned out that tt_min_abs is minimization, not absolute value minimization as said in comments, so I removed it.

I also fixed bug with tt_min -- sometimes first tt_max_abs give wrong answer and hence tt_min return maximum value instead of minimum.
Example:
Assume you have a tensor with {1, 2. ..., 100}. Assume your first tt_max_abs returned 30. You subtracted 30 from the tensor and got {-29, ..., 70}. Now second tt_max_abs would not be able to find correct minimum.