Fix definition of "coordinate vector"

[cmaumet]

This PR fixes the definitions of "coordinate vector" and "coordinate vector in voxels" to specify in which units each coordinate vector is expressed:

• nidm:'coordinate Vector': A vector with one number per dimension. The first element corresponds to the 'Coordinate' along the first dimension measured in world units (e.g. 'mm'), the second element to the 'Coordinate' along the second dimension etc.
• nidm:'coordinate Vector In Voxels': A vector with one number per dimension. The first element corresponds to the 'Coordinate' along the first dimension measured in map elements (e.g., pixels, voxels, vertices, or faces), the second element to the 'Coordinate' along the second dimension etc.

in replacement of:

• nidm:'coordinate Vector': A vector with one number per dimension. The first element corresponds to the 'Coordinate' along the first dimension measured in map elements (e.g., pixels, voxels, vertices, or faces), the second element to the 'Coordinate' along the second dimension etc..
• nidm:'coordinate Vector In Voxels': Coordinate along the first dimension in voxels. (This definition needs to be re-worked as this term was renamed from coordinate1 to coordinate in coordinate1, coordinate2, coordinate3 -> coordinate #270)