Groan.
More stuff.
More compilation fixes, before ordering of times.
SFINAE
Move is_quantity.
Ints and doubles are quantities.
Merged with #147.
Use the new names.
More name substitutions.
Improve is_quantity.
More compilation fixes.
Tests running.
Tmax is an instant.
No warning.
Debug.
After eggrobin's review.
Merge pull request #148 from pleroy/SpeedTime

Use Velocity to represent the speed and Instant to represent the time.