Enable sequential processing

[igorinov]

Sequential processing can be used to process data from multiple independent sensors with different sampling rates.
It also eliminates the matrix inversion operation.

[rlabbe]

This looks great, and I'll be happy to incorporate it.

Before that, do you have any unit tests you could incorporate? I don't like checking in new functionality without passing tests. At a minimum the test provides a hint on how to use the method, and that is important since this method is not (yet) documented in the companion book I wrote.

Also, if you have a good paper or textbook citation to add to the docstring that would be nice, but I can did something up if not.

[igorinov]

Thank you! Added a unit test.

Anderson, Brian D. O.; Moore, John B. (1979). Optimal Filtering.
Reprint (2005) by Dover Publications, NY pp. 142-147 ISBN 978-0-486-43938-9

[igorinov]

Another test for the same algorithm implemented in Java.

https://github.com/igorinov/variometer/blob/a08160a45f623bbe47083f3efb7365f93775fb52/common/src/test/java/com/igorinov/variometer/common/KalmanFilterTest.java#L94