Fix contact_thresh_neg value for the lift

[hello-chintan]

The lift contact_thresh_neg value is not respected when supplied with move_to, move_by command because for the lift a sane value for this argument would still be positive. However, in HelloController, the i_contact_neg value is always converted to a negative value by multiplying the absolute with -1. Hence, the guarded contact comparison for contact_thresh_neg always happens with a negative value at the controller level which for the lift is achieved only when a very high force is applied.

The fix in this PR gets rid of the extraneous operation with -1 as it’s not necessary if the user sets the contact_thresh_neg value with the correct sign.

[aedsinger]

This is indeed a bug. However, the orginal intent was to safely handle the case when i_contact_neg > i_contact_pos. This would happen for motors that were setup up backwards. Checking the stretch_params it seems that it should be OK to make this change. However, I'd recommend modifying the firmware to set the the contact threshold to zero if this happens:

if (g_eff_pos<g_eff_neg){g_eff_pos=0;g_eff_neg=0;}

after line 820. Test the code to make sure this happens. Conversely, this could be handled at the Stretch Body level (with the advantage of error/warning reporting)