Fixed bug in Entity fields value() calling len() on None

The value function for Entity and Multi-Entity fields had the possibility of
calling len() on a None value.  This is because it was referencing the wrong
variable in an if statement.
Added an example SgProject entity with util functions for Seqs and Shots

Added an example SgProject Entity with util functions for getting a list of
Shot and Sequence Entities for the project.  Also added functions for getting
just the names of Shots and Sequences for a project.