Drop plural (s) from TrackMeasurements and TrackParameters

A small naming issue was overlooked for new tracking EDM classes. Namely single object were named with plural. Since we are going to live with these classes and have not written single event yet I decided to make the names alike for other objects.

A guide for review: have a look at individual commits, they contain files moving & classes renaming.

Related to: ATLIDTRKCP-364