Minor changes to add nullability and __kindof and change messages to properties

[nivekkagicom]

These changes add the nullability macros NS_ASSUME_NONNULL_BEGIN and NS_ASSUME_NONNULL_END to the headers and source files and adds the nullable keywords where needed to eliminate warnings. The changes also change cell and setCell messages to be properties and uses __kindof to eliminate other warnings resulting from multiple possible messages for cell.