[ENH] concat operation on distributions

[SaiRevanth25]

Reference Issues/PRs

#340

What does this implement/fix? Explain your changes.

Implements the concat operation on distributions.

See #340 (comment)

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[fkiraly]

changed it to draft, until it is not finished

[fkiraly]

Great! Pattern looks good.

I assume this is work in progress, so some comments:

• the __init__ may need to have an index and columns argument.
• implementing mean etc is the old, deprecated extension pattern. Now you need to implement _mean etc, see extension_templates/distributions.
• there are more methods to implement, and get_test_params should be set.