Shorter CoordinateSystem names

[cmaumet]

This issue is here to keep track of comments provided by @satra on #123:

• can we keep URLs as short as possible? as such could create further hierarchies for the MNI152Nonlinear[a|b|c][Asymmetric|Symmetric]CoordinateSystem and thereby reduce the term name.
• i think "CustomCoordinateSystem" should be parallel to worldCS. not that it matters in brain imaging now, but you could fix a coordinate system in space, what if they put a portable scanner on the international space station or as part of the one way trip to mars in 2024 :)