RF: Rename 'clfcv' command to 'crossval'

Not really about classifiers alone.
NF: Add 'dsinfo' command to describe a dataset's content
RF+BF: Refactor uniquemerge2literal to support multi-dim attributes

This also removes the custom conditional wrapping fo return values into
lists to imply a conversion scheme. Now this function always returns
sequences if it does its job. However, the special case of returning
None for empty sequences and the input for non-sequences are maintained
for backward-compatibility.

Add a dedicated test.
ENH: Use appropriate attr merge fx in mkevds command
ENH+NF+BF: cmdline helpers update

- Uppercase filenames now possible for custom partitioners
- Custom partitioning attributes for built-in partitioners possible
- argparse type conversion for error functions
ENH: More complete draft of a 'crossval' command