Tensor space

[Tragicus]

Motivation for this change

Binary and indexed product tensor spaces.
@CohenCyril I have little recollection of what we did, so I have no idea what the status of this is. In particular, I see the universal property of the binary tensor space but not the general product one.
@mkerjean FYI

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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As a rule of thumb:

• PRs with several commits that make sense individually and that
  all compile are preferentially merged into master.
• PRs with disorganized commits are very likely to be squash-rebased.

Reminder to reviewers

• Read this Checklist
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[mkerjean]

This looks nice, thank you for making this PR! Could you tell me what the \pi_Q map represents? Could you point me to the universal property for the binary tensor product within the code, so that I can start from here? More generally, happy to help on this PR, tell me what I can do.