gawebb-dstl

I've started on the issue here -> 502f4c80198774c16b13893667ce8d856a2619dc. Could you please have a look and see what you think

In the first instance I'll look to Basic Convex Combination from [here](http://fusion.isif.org/proceedings/fusion99CD/C-134.pdf). With the implementation given below:   @sdhiscocks any thoughts on where this should go inside the SS...

Thoughts on the approach @etfrogers-dstl

> > Add option to skip detections in `MultiMeasurementInitiator` with a non reversible measurement model > > The multi-measurement initiator doesn't have any calls to the inverse function of the...

I'm not sure if `SimpleMeasurementInitiatorWithCovar` adds much, let me know what you think. If you think `SimpleMeasurementInitiatorWithCovar` is worth having, I'll add some tests and documentation