v0.5.0: Fast mesh ops with Rvcg and many fixes

Version 0.5.0

• Breaking change: remove long deprecated parameter 'colormap' from all vis functions. Use the 'makecmap_options' parameter instead.
• The function vis.color.on.subject now supports the 'style' parameter supported by other vis functions.
• Add new highlight.* functions which are useful mainly for debugging.
• Fix accidental printing of rglactions to console when calling vis functions.
• Add new 'export' function, an alias for 'vis.export.from.coloredmeshes' so you have to type less. Updated vignettes and example notebooks accordingly.
• Add new entry on vector graphics conversion to the 'fsbrain FAQ' vignette.
• Add surface.curvatures function that can compute k1 and k2 (requires optional dependency package 'Rvcg').
• Add functions to write group standard space per-vertex data: write.group.morph.standard and write.group.morph.standard.mf
• Add new and more convenient volume visualization function: volvis.lb is a convenient wrapper around volvis.lightbox.
• Add new function demographics.to.qdec.table.dat that can convert a demographics dataframe from an Excel or CSV file to qdec.table.dat format and write the additional factor level files.
• Add new sjd.demo() function, a shortcut to download optional demonstration data and get the path to the downloaded subjects dir.
• Add vis.subject.pre function, a new high-level function for visualizing preloaded data (see #31)