Rtsne 0.15

• Substantial speed increase by fixing the possible embedding dimensionalities to 1, 2 or 3
• Clarification of the licensing of different parts of the package
• Support for using, more efficient, partial PCA (by Daniel Wells)
• Made the normalization optional and the normalization function used available (by Aaron Lun)
• Support for using precomputed nearest neighbour matrices (by Aaron Lun)
• Added OpenMP support
• Default verbose value is now the global setting (by Richard Cotton)
• Added a NEWS.md file to track changes to the package.