Analytical derivatives for eight-chain model

[miguelmaso]

Hi @jmartfrut and @RogelioOrtigosa , thanks for sharing the analytical derivatives. Here it is the implementation.

I've added a tool for easing fast benchamrks. In the future it could be moved to a new file. A possible output is

Analyitical ∂Ψu  |   93.572 ns (0 allocations: 0 bytes)
Numerical ∂Ψu    |   168.590 ns (0 allocations: 0 bytes)
Analyitical ∂Ψuu |   295.935 ns (0 allocations: 0 bytes)
Numerical ∂Ψuu   |   4.414 μs (85 allocations: 3.27 KiB)

Changelog

• Added derivatives implementation
• Added test for eight-chain model
• Removed warning related to redefinition of a variable
• Added function prototype for benchmarking (it should be moved to a new file)