Package visibility, sanitizing code inclusion (rootness checks)

This is a small incremental release:

• making the project API visible from outside the package,
• providing saving and restoring tensors,
• preventing some user bugs by "rootness checks" (regarding when code pieces are included with tensor references).

From the changelog:

Added

• Tensor parameters saving and restoring, Ndarray saving and restoring.
• An operation outer_sum: like einsum but simpler, addition everywhere.

Changed

• Tweaks to make the project usable as a package (external library).
• Sanitizing code inclusion via code roots management: Tensor.consume_forward_code and consume_backprop_code, (optionally but by default) used from Train.

Fixed

• Shape inference in presence of non-0 fixed indexing inside einsums was broken (because actually not implemented).
• Incompleteness of shape inference for slicing was leading to inferring shapes with no axes: constraint generation was intended to raise a shape error instead. Proper fix coming in 0.3.2 will make slice shape inference complete.