Adding `concat` functionality

[MarcoGorelli]

In the last call, it was brought up that concat was already discussed, it just hadn't been implemented. However, I can't find it in this repo, nor in the notes - does anyone know where it might be?

If not, here's what I suggest putting together:

def concat(self, other: Sequence[DataFrame]):

So

df1.concat([df2, df3, ..., df10])

rather than

concat([df1, df2, ..., df10])

(the issue with the latter being - how would top-level methods like that be accessed?)