Integro-differential equation PINN solver benchmark

[killah-t-cell]

Would be great to have a benchmark for IDE PINNs.

[killah-t-cell]

Specially since IDEs are much slower than PDEs in NeuralPDE (training sometimes goes from 1 minute to 1 hour by adding one integral). This is to be expected as we discussed, but it would be cool to have a benchmark as a way of checking speed and accuracy against other methods.

[Spinachboul]

@killah-t-cell Then it would require benchmarks which have some kind of noise factor. But we cant have muti-dimensional problems