Revise plotter's arguments (blocks, attributes)

Some plot methods in Plotter accept only a single block name, and some allow a list of block names. For example, one can plot together correlations of DPs and PAs, but distributions, distributions2d and contours2d accept variables only from one of the blocks at a time. Could this be amended to allow variables from multiple blocks?

Also, the necessity to specify a block even when variable names ("attributes") are given is superfluous - one can easily retrieve block names for given data objects. Block name could instead be used as a shorthand for all variables in a given block, and "attributes" argument could be used primarily if one wants to pick specific variables.

• contours2d: allow combining variables from various blocks
• contours2d: allow specifying attributes only or blocks only
• contours2d: only works if dataset is provided (datablocks in datamodules are TransformableDataBlocks and don't contain data, which is needed)