Delayed operators

[yaxxie]

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Before submitting this PR, please have a look at the below checklist so that we know more about your PR. Please also reference any relevant issues from the issues page if this PR is intended to address one of those.

What does this PR do?

• Adds support for some commonly used operators (binary) in Delayed expressions
• Adds tests for these cases
• Adds support for unary operators, but no defaults other than minus.
• Tests demonstrate that it is possible to plumb an own operator if really desired

Checklist

• [x] Your code builds clean without any errors or warnings
• [x] Unit tests have been added for core changes
• [x] julia --project -e 'using Pkg; Pkg.test()' has been run locally and passes
• [ ] Documentation has been updated with corresponding changes (if applicable)

[kainkad]

@yaxxie thank you so much for this amazing PR to bring the transformations in. I'll double the spaceprint but it looks good as it is. I might just add few more test scenarios as the ones on the PR are currently showing transformations where left and right or 'a' and 'b' are HP functions whereas we have few use cases where the left operator would be a number but I can see it's implemented in delayed operators with the 'Real' dispatch so it should just work. I will sort out documentation in a separate PR with some visualisations for the users to show these transformations.

[yaxxie]

I'm happy to add the extra tests

[yaxxie]

a7c60025bdc20276ba2f9ff9d6f05a47037ad6cb adds a few quick and dirty scalar operation tests -- it doesn't check both left and right side of binary operators, which would be an issue if the operator was not registered for one of the other sides, but its also a super quick patch if it was. Let me know if you need anything else for this PR