Clarify where in $apply parameter aliases are allowed

[HeikoTheissen]

The Data Aggregation spec is mostly silent on this point.

• According to the core spec, they are allowed in functions like Aggregation.isancestor.
• Since the filter or orderby transformations are defined in terms of the corresponding system query options, they are allowed here.
• Should be analogous for compute, but is not, because this speaks only of a "common expression".
• What about topcount etc.?

[ralfhandl]

Re compute:

• commonExpr includes firstMemberExpr
• which includes inscopeVariableExpr
• which includes parameterAlias

Parameter alias are thus allowed, better add a test case if one doesn't exist yet.

[ralfhandl]

Re topcount etc.:

• if they allow commonExpr (which topcount does), they allow parameter aliases

Again a test case would be helpful if one doesn't exist yet.

[HeikoTheissen]

Seems the ABNF is a better source here than the specification.

[ralfhandl]

We may want to change that.