- Added documentation and renamed some constants.
- Turned on the acts tracking geometry.

This reverts commit 78e317ee

The segmentation should be X**Z**. This is because of how the trd is constructed (with z along the normal y direction).

	modified:   compact/subsystem_views/calorimeters.xml

	modified:   view20.yml
	modified:   view3.yml
	modified:   view7.yml

 - Color scheme is separate from the display.
 - This allows a coherent color scheme to be used.
 - Also added a standard display.xml and display_detailed.xml (currently
   just a copy)
 - A new color scheme should start with colors2.xml which is currently a
   copy of colors.xml
 - created grouped subsystem includes.

Also began adding subsystems:
 -  compact/subsystem_views/pid_only.xml
 -  compact/subsystem_views/tracking_only.xml
 -  compact/subsystem_views/vertex_only.xml

 - using "athena"
 - this avoids any clashing with the reference detector libraries.

 - Looks like some overlap issue with the face endcap layers and the
 barrel layers' end-face .

Paying attention to following items:
* Overall parameterization -- will use for optimization
* Constant naming consistency
* Documentation via the comments
* Added new ci job which concatenates all the `<comment>` tags into `docs/detector.md`
* All comments should now be formatted to fit the appropriate section in the markdown file.

- The outer tracker barrel's 2 layers are commented out
- The subsystem is then properly constructed in the acts geometry
- There is probably a small bug in this part of the barrel which is
inconsistent with the way ACTS constructs its tracking geometry