Define matrix for categorical measurement model

[oharrald-Dstl]

Currently, the model uses transpose of emission matrix. This is not correct. I've left this as an issue in the hope of a discussion.

https://github.com/dstl/Stone-Soup/blob/14db74f6557a46ff3ddd8db12e2d23480ecdfee2/stonesoup/models/measurement/categorical.py#L63