Compute r12 using linear interpolation.

def _fwhm_side_lineal(uu, vv):
    '''Compute r12 using linear interpolation.'''
    res1, = np.nonzero(vv < 0)
    if len(res1) == 0:
        return 0, 1  # error, no negative value
    else:
        # first value
        i2 = res1[0]
        i1 = i2 - 1
        dx = uu[i2] - uu[i1]
        dy = vv[i2] - vv[i1]
        r12 = uu[i1] - vv[i1] * dx / dy
        return r12, 0