Deprecate relative PNG path in favor for relative data path

`relativePngPath` should be replaced by `relativeDataPath` as suggested in https://github.com/nschloe/matlab2tikz/issues/181#issuecomment-25689398 .
The old syntax is deprecated but still works. Internally only `relativeDataPath` is used; when it is not provided, but `relativePngPath` is, the user is warned that this this usage will stop working in the future.

As cherry on the cake, I introduced a small simplification in counting the number of PNG/table files, such that less logic is needed in the counting code.