Large-scale API indexing re-organization:

Large-scale API indexing re-organization:

 * Moved FileExists() to BamAux.h so that all API classes have access to its functionality.
 * Created 2 'factory methods' in BamIndex.h to return a BamIndex subclass, depending on client\'s specified PreferredIndexType & on what files actually exist on disk.
 * Renamed BamDefaultIndex as BamStandardIndex.  Hopefully this name should be a clearer description going forward than BamDefaultIndex, since the standardized index may not always be the 'default' in every situation.