Deprecated `relativePngPath`

The `relativePngPath` parameter is deprecated and the user is warned for this.
Everywhere, `relativeDataPath` is used.

When the user only provides `relativePngPath` and no (or an empty)
`relativeDataPath`, `relativePngPath` will be used to populate
`relativeDataPath` and again the user is warned this will break in
a future release.
PNG and data file counters populated by default

Populating by default with 0, so in generating the files,
no logic is needed, we can just increment the value before
constructing the file
Merge pull request #290 from egeerardyn/deprecatePngPath

Deprecate relative PNG path in favor for relative data path