In SELECT(): further rewrote, and festooned with comments, the code

In SELECT(): further rewrote, and festooned with comments, the code
that verifies that every column in the SELECT clause comes from a
class in the FROM clause.

Also, for readability: reversed an "if" test that treats functions
differently from classes.

git-svn-id: svn://svn.open-ils.org/ILS/trunk@12069 dcc99617-32d9-48b4-a31d-7c20da2025e4