Support for tensors

[mykelk]

This package currently supports vectors and matrices like this:

@satvariable(z[1:5,1:5], Bool)

But, it doesn't currently support higher dimensional tensors. For example, this fails:

@satvariable(z[1:5,1:5,1:5], Bool)

[mykelk]

In order to support this, I think this function would need to be generalized:
https://github.com/elsoroka/Satisfiability.jl/blob/main/src/smt_macros.jl

[elsoroka]

I merged the PR! Remaining tasks (that I can take on next week) are to:

• Add unittests for this functionality
• Update the documentation to mention it + add your example
• Add it to the Release Notes page
• Make a new release