Improve .sub() -- don't use .reshape()
make_unique is private
Make a typedef for uint / index type
Unify Element iterators through baseiter
Improve matrix multiplication

Add dedicated methods for row and column iterator
Rename .sub() to .at_sub() to avoid conflict with `-` operator method
Improve doc for Slice
Rename new to from_elem
Rename apply_slice to islice
Add test for Array of char
Array and Slice doc
Add simple tests for operators
Use doc generation with custom style