Statistics.stack: Move to statistics module

[pavlin-policar]

Issue

hstack and vstack existed in Orange.data.util, however all the other functions that handle sparse and dense data are situated in Orange.statistics.util.

Description of changes

Move functions and tests to Orange.statistics.util module to keep all functions that deal with sparse/dense compatibility in same module.

Based on #2572.

Includes

• Code changes
• Tests
• Documentation

[kernc]

Add

from Orange.statistics.util import hstack, vstack  # back-compat

[nikicc]

Hmmm ... I like that we have everything together but importing vstack and hstack from Orange.statistics.util seems strange to me.

Perhaps, should we introduce a separate module for this sparse/dense compatibility methods? Some other methods from Orange.statistics.util could be moved there.

[pavlin-policar]

I agree, Orange.statistics.util is probably not the best place for this. I think Orange.data.util would be a much better module, or something new altogether.

Of course, we'd have to move a large number of functions from there.

[pavlin-policar]

This had conflicts and it would be easier to simply start fresh from master, since the changes in this PR were quite trivial.

We had decided that it would be strange to have common functions (such as vstack and hstack in a module named statistics; a different module would have to be made where we move all the methods that are essentially wrappers around sparse and numpy functions. This introduces backwards compatibility issues.