[Fixes #322, #330] scatter: 'mark size=' if one size for all markers

I am always specifying `mark size=` if the size is unique for all scatter points. This means that the rescaling is always performed. 

However, it seems #316 is open again, since I cannot reproduce with the acid tests the graphs that @nidrosianDeath shows in one of the last [comments](https://github.com/nschloe/matlab2tikz/pull/316#issuecomment-35383893) and (the previous one).