MbTiles(Overlay) zoom interpolation 

Ok, as discussed now a clean pull request.

This will, when no (overlay) MbTiles data is found, modify the database request until it finds a tile.

If no data is found it gives up and as before no tile is returned.

If found, the zoom level difference is checked. For now this will only work for a zoomleveldifference of 1, but is prepared for a larger difference. According to that difference the data (pixels) are cut out of the available tile (now working for one level, has to be reviewed for higher differences) and these pixels are scaled up (working for any difference)  and returned as a tile.

Anyway, as for a difference of 2 or 3 only 16 resp. 4 pixels would be upscaled, a limit of 2 different levels would be reasonable, according to me. Or we leave it as it is, as one level is working out.

For zoom level differences larger than one,as before, no tile is returned, hence no MbTiles overlay.