Cleanup b-tagging track info accessor

We use a BTagTrackAugmenterAlg to store information IP based taggers can only calculate with a magnetic field. There's also a BTagTrackAugmenter (renamed BTagTrackIpAccessor here) which reads this information off tracks and presents it to the b-tagging code.

This merge request fixes a few things:

• Try to avoid using any more track decorations than we have to. This MR removes two redundant variables. In doing so it changes the values we see by roughly 1 part in 1e7. This won't have any effect on the physics output but things won't be binary identical.
• rename d0 method to absD0 (part of implementing the above means this changed) (recovered the sign instead)
• remove all the decorators in this class: make it a pure accessor (need to do this later, methods are still used)
• Rename the class to something more appropriate.