readability

[murphyk]

• [x] need to explain that C in lds.kalman_step can be a constant matrix or a function of time, as in this linreg example
• [x] it would be clearer to rename xt to obs or observed
• [x] you say x, y = xt in this KF code. What does this mean? What is y?
• [x] you say xt, obs.= xs in this EKF code. I think this should be obs, inputs = xs. Need to explain signature of fx and fz!

[gerdm]

Fixed in latest push