v0.7.0 "Memories of Ice"

Arraymancer v0.7.0 Jul. 4 2021 "Memories of Ice"

This release is named after "Memories of Ice" (2001), the third book of Steven Erikson epic dark fantasy masterpiece "The Malazan Book of the Fallen".

Changes :

• Add toUnsafeView as replacement of dataArray to return a ptr UncheckedArray
• Doc generation fixes
• cumsum, cumprod
• Fix least square solver
• Fix mean square error backpropagation
• Adapt to upstream symbol resolution changes
• Basic Graph Convolution Network
• Slicing tutorial revamp
• n-dimensional tensor pretty printing
• Compilation fixes to handle Nim v1.0 to Nim devel

Thanks to @Vindaar for maintaining the repo, the docs, pretty-printing and answering many many questions on Discord while I took a step back.
Thanks to @filipeclduarte for the cumsum/cumprod, @Clonkk for updating raw data accesses, @struggle for finding a bug in mean square error backprop,
@timotheecour for spreading new upstream requirements downstream and @anon767 for Graph Neural Network.