Implement more group_by functions

[dreadatour]

This is a follow-up issue for the #228.

We still have implement the following group_by functions:

• first
• last
• std
• var
• min - for not int/float should return None is None in a list, otherwise - any value(like max([None, None, "Cat", "Dog", None]) --> None)
• max - for not int/float should return any value but None (like max([None, None, "Cat", "Dog", None]) --> "Cat")

[dmpetrov]

We need to update the code to use new func module only.

Yes, it should be imported from top level from datachain.func import min, concat

PS: it looks like there are 3 issues in this one :)

[dreadatour]

PS: it looks like there are 3 issues in this one :)

🤔 Indeed. Let me split it.

[dreadatour]

PS: it looks like there are 3 issues in this one :)

🤔 Indeed. Let me split it.

#524 + #525