More file formats and improved MGH header support

• Add support for reading surfaces in FreeSurfer ASCII surface format (integrated into read.fs.surface)
• Add high-level functions 'read_nisurface' and 'read_nisurfacefile' to read surface files in different formats
• Add support for writing surfaces in FreeSurfer ASCII surface format: 'write.fs.surface.asc'
• Turn return value data structure of read.fs.mgh into 'fs.volume' class and implement 'print' and 'is' S3 methods
• Support reading the ASCII version of FreeSurfer curv format files (integrated into read.fs.curv)
• Support reading the FreeSurfer weight or 'w' format files (read.fs.weight), both ASCII and binary subtypes are supported
• Add support for reading binary patch files
• Add support for reading ASCII patch files
• Fix skipping header bytes while reading mgz files (issue #10)
• Read fov MR acquisition parameter from MGH header, make mr_params parameter of write.fs.mgh length 5 from 4
• Compute dof and various other header fields from MGH header data, add new functions to derive matrices
• Add a third vignette that explains how to compute the MGH header data