computes spatial-temporal parameters for any walking condition based on 'Defining spatial parameters for non-linear walking' Gait & Posture 23 (2006) Huxham et al. ARGUMENTS FS1 ... coordinates of 1st ipsilateral foot strike FS2 ... coordinates of 1st contraleral foot strike FS3 ... coordinates of 2nd ipsilateral foot strike leg_length ... optional. Length of leg, used to normalize paramters RETURNS StrideLength ... Stride Length in same units as input units StepLength ... Step Length in same units as input units StrideWidth ... Stride width in same units as input units