A Unit.T is a sparse vector with integer entries Each map n->m means that the unit of the n-th dimension is given m times.
Example: Let the quantity of length (meter, m) be the zeroth dimension
and let the quantity of time (second, s) be the first dimension,
then the composed unit m/s^2 corresponds to the Map
[(0,1),(1,-2)].
In future I want to have more abstraction here, e.g. a type class from the Edison project that abstracts from the underlying implementation. Then one can easily switch between Arrays, Binary trees (like Map) and what know I.