Correct handling of reading courses
Correct handling of reading courses

This amends commit d90b0a3334b96268c0f30745bc257c5e81737f8f
"Preventing importing of courses with invalid catalog numbers",
which considered courses with short or undefined catalog numbers to be invalid.
However, the definition of short catalog numers mistakenly included reading course numbers.

sebthedev/PrincetonCourses,sebthedev/PrincetonCourses