cleanup of cellml specification units
There is some ambiguity it seems about the intended interpretation of some aspects of the CellML specification surrounding units. References include:
Meeting Minutes 13th November 2006
http://www.cellml.org/pipermail/cellml-discussion/2006-November/000367.html
http://www.cellml.org/pipermail/cellml-discussion/2006-October/000292.html
We need to gather together test cases we have for demonstrating correct declaration and software behaviour.
