Add more convenient method(s) to IIonReader for determining whether an annotation is present

[pbcornell]

The IIonReader interface currently provides:

IEnumerable<SymbolToken> GetTypeAnnotations()

which isn't particularly convenient when checking to see if a specific annotation is present. Also note that ion-java's equivalent of this method is called getTypeAnnotationSymbols().

Something like string[] GetTypeAnnotations() or bool HasAnnotation(string annotation) would be more helpful.

[tgregg]

Once this is added, use it in the Sparse Reads example in the cookbook, replacing the inline implementation that is currently there.

[pbcornell]

To clarify, I propose the following changes to the IIonReader interface:

• rename the existing GetTypeAnnotations() method to GetTypeAnnotationSymbols()
• add string[] GetTypeAnnotations()
• add bool HasAnnotation(string annotation)

Note that the first two bullets represent breaking changes.

[tgregg]

Closed via #77.