2.7.0 Fill and form methods; improved docs

• Added fill and form methods, which take nested curly brace initializer lists to set values or create a new rarray based on the initializer's shape.
• Added a slice method. Slicing is possible only in the first dimension, to preserve a contiguous layout of the array elements.
• Better documentation, with tutorial now in markdown and developer documentation that can be generated with doxygen.