add unit times() function

* double * unit and v.v. returns quantity
* unit * quantity returns quantity
* don't use transpose@double(x.average) just use transpose(x.average)
 since exactly same thing, but more confusing - need to break it out
 becasue transpose calls subsref and subsasgn, which can't handle
 proprties of quantity that need referencing and indexing
* fix only display aliases if there exist
* remove suppression of "growing each iteration" warning
* add comment that subclass of double requires superclass constructor call
* coerce variance to double from numeric
* make empty cell array default for aliases property of unit