Grammar fix for CENTROID

[Zarquan]

From an email discussion with Markus

I had to dig into the ADQL grammar in another matter, too: CENTROID.

For one, pgSphere so far doesn't support computing centroids (except
for circles and points, which is lame), so I suspect nobody supports
this properly. I, at least, don't, and now try to give a sensible
error message.

But while digging into this I noticed back then I've quitely
sanitised the grammar, and I think we should fix this "upstream" now.

Currently, we have

<centroid> ::= CENTROID <left_paren> <geometry_value_expression> <right_paren>

<geometry_value_expression> ::=
     <value_expression_primary>
   | <geometry_value_function>

<value_expression_primary> ::=
       <unsigned_value_specification>
     | <column_reference>
     | <set_function_specification>
     | <left_paren> <value_expression> <right_paren>

So, something like CENTROID(47839) or CENTROID(COUNT(*)) is
grammatically ok, and I suspect that was not what people had in mind.
I suspect what they wanted was to allow a column reference. So, I
think what we want is

<geometry_value_expression> ::=
     <column_reference>
   | <geometry_value_function>

The other alternatives in <value_expression_primary> don't really
make sense for any place <geometry_value_expression> turns up in.

On the other hand, what I've been running all these past years was
essentially

geometryExpression = box | point | circle | polygon | region

geometryValue = columnReference.copy()

centroid = (CaselessKeyword("CENTROID")("fName")
    + '(' + Args(geometryValueExpression) + ')')

geometryValueExpression = geometryExpression | geometryValue | centroid

I can't promise I really thought the inclusion of centroid in
geometryValueExpression, but I'd support including it in
<geometry_value_expression>, too. It's probably not deadly
important, but it'd allow stuff like

1=CONTAINS(CENTROID(coverage), CIRCLE('', 10, 2, 3))

or similar.