* fixes#3493 allowing math.unit(5), which would now fail because for values of MathNumericType and MathCollection, the second parameter wasn't optional
* Added tests, and as a result of that, fixed the definition of unit(value: MathCollection): Unit[] where no units seem to be allowed
* Changes as requested by josdejong
* math.add and math.multipl type fixes and added tests
* fixes#3494: math.unit(m).to(math.unit(cm)) is now ok for typescript