The S3 release

• Fix input data check in write.fs.mgh
• Turn return value data structure of read.fs.annot into 'fs.annot' class and implement 'print' and 'is' S3 methods
• Turn return value data structure of read.fs.label into 'fs.label' class and implement 'print' and 'is' S3 methods
• Support 'fs.label' instances as parameter for 'write.fs.label'
• Turn return value data structure of read.fs.surface into 'fs.surface' class and implement 'print' and 'is' S3 methods