WCPS - NEW - build query tree for optimization before generating rasql query

[Bang Pham Huu]

petascope would need is to first build a query tree for each coverageExpression, which has:

• Pointer to parent expression node
• Pointer to children argument nodes, and an attribute to distinguish its type.

The Handler classes could be adapted for this purpose, so:

• Instead of calling their handle method directly during the parsing, petascope would construct the Handler object and return that from each visit method.
• Afterwards, calling rootNode.handle(..) would have the same effect of returning a WcpsResult with the rasql. But it would also be possible to traverse the tree and modify it, e.g. push the Scale handler or change its arguments, before calling handle().