Cross-nest queries in NestedFrame.query should be prohibited or split

[gitosaurus]

Bug report

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Processing queries across base and nested columns is not possible in NestedFrame.query today, so expressions of this kind are detected and prohibited. However, it does not presently prohibit queries that go across two different nests:

nf.query("n1.c < 0.5 and n2.d > 1e-7")

The above will "work", but only selects the nested rows in n1, simply because n1 is earlier in the string.

The solution is to treat such cross-nest queries exactly the same as queries that cross base and nested columns: error out today, and as split queries, in the future.