new namespaces are
international ( combination of us and imperial + Canada etc)
iau (https://www.iau.org/
imperial ( old imperial units)
typographical ( for sizes of printing fonts etc)
These namespace are based on some research , mainly on wikipedia.
Look in src/include/units/physical/si/length.h to see links to see the references to documentation justifying the change.
Unfortunately there are 3 foot units for example, an old imperial version, an old us version and an international version, which is more recent and attempts to unify the two previous ones. All versions have slight changes in value, so I opted to use the international version
The main change in the layout is that inch,foot and yard have been moved from us to international.
With this modification, I also modified the physical/us/area.hpp, volume.hpp and volume.hpp headers to refer to the international units.
This may not be correct, but if the modified us::foot (rather than international foot is used as a basis for these units, then
there is a ratio integer overflow during compilation, probably due to taking 3rd power of a ratio. After this commit. I will try to show that on another branch.
