improvements for the PintIndex

[keewis]

This is a list of tasks we skipped in #163:

• figure out how to properly recreate an index from changed coordinates (recreating indexes after conversion #269)
• modify the IndexSelResult returned by the wrapped index' .sel
• implement concat
• implement stack and unstack
• implement join
• implement reindex_like
• figure out how to best compare indexes with compatible but not identical units

For each of the methods, we also need to see if we can deprecate any of the workaround methods in the .pint namespace.