BUG (unexpected behavior): Class should not immediately execute `process_query` when instantiated with a query.

[simeonreusch]

Currently, e.g. the PostgreSQLQueryProcessor (this applies to all QueryProcessors) immediately executes process_query when a query parameter is parsed.

This is unexpcted and should be changed, so it always works as follows:

a)

qp = PostgreSQLQueryProcessor()
qp.set_query(query)
qp.process_query()

or b)

qp = PostgreSQLQueryProcessor(query=query)
qp.process_query()