Move all dimensions operations to Picture::Transformations

All point and string arguments can tage the "x20" or "20x" arguments.
Points get 0 interpolated, height or width get the width or height of
the crop_size or of the original image.

Upscaling is not allowed unless upscale is set to true at the end of method
calls.
We should use `params[:upsample] == 'true'`
Because we want `@upsample` only to be true, if one passes `'true'` as param. Your code also equals true, if one would pass `upsample = 'false'`
Rails does not convert `"true"` and `"false"` in params to `Boolean`, the stay `String`