Transforms, more mesh formats and better GIFTI support

Version 0.1.11

• export readable.files function
• support reading and writing meshes in mz3 format
• support writing PLY2 ASCII mesh format
• support reading mesh in BYU format
• support automatic conversion of quadrangular BYU meshes to triangular upon import
• add support for reading GIFTI format labels
• add support for reading GIFTI format annotations
• add support for writing GIFTI format labels
• add support for writing GIFTI format annotations
• export rotate2D and flip2D to namespace
• add support for reading transformation matrices from xfm files: read.fs.transform
• rename all optional data functions: replace optional_data with opt_data in function names to prevent clash with fsbrain names