Add Tensor wrapper for jacobians in DualArrays

[max-vassili3v]

This PR adds a new Tensor struct, with constructors and arithmetic/broadcasting. Dual and DualVector were changed to have jacobian and partials be a Tensor. Existing unit tests were adjusted where necessary and a new file was added for testing Tensors in isolation.

Next steps:

• Example of 2nd order differentiation/hessian using DualArrays/Tensor
• Implement contraction and sparsity using SparseArrayKit.jl and TensorOperations.jl

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.34%. Comparing base (5761344) to head (2e4e543).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #28       +/-   ##
===========================================
+ Coverage   70.87%   90.34%   +19.46%     
===========================================
  Files           5        5               
  Lines         103      176       +73     
===========================================
+ Hits           73      159       +86     
+ Misses         30       17       -13     

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[dlfivefifty]

Rename Tensor to eg TensorOperator?

[dlfivefifty]

Or probably better ArrayOperator. Probably not ArrayMatrix. Or maybe just leave it Tensor for now.