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Select aggregates that a projection selects directly (#198)

Translating algebra to SPARQL dropped an aggregate when a projection
selected the aggregate's variable directly, as in
Project(Group(BGP, [?s], [?var0 := COUNT(?o)]), [?var0, ?s]). This
generated SELECT ?var0 ?s ... GROUP BY ?s, which loses the COUNT and is
invalid SPARQL 1.1 (section 11.4), so the parser rejects it.

translateAlgProject only turned a projected variable into a projection
expression when an Extend bound it. A Group's aggregate was substituted
into Extend and HAVING expressions, but never into the projection, so a
projected aggregate variable without an Extend was printed bare. A
SELECT now projects such a variable as (aggregate AS ?variable).

toAlgebra never produces this form: it gives each aggregate a fresh
variable, and binds the selected name to it through an Extend. The
shorter form means the same query, though, and other tools build it,
for example when they send a Group to a SPARQL endpoint on its own.
Algebra-only fixtures cover it, in a new sparql-1.1-algebra-only suite
that pairs algebra JSON with the SPARQL it must translate to.

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