Add ipu specialised implementations of special functions used in experimental.integrals

[hatemhelal]

The following special functions are used in the evaluation of electron integrals following the THO methods:

• factorial: could we just use the scipy impl? Not available in the v0.3 JAX that we are pinned to.
• factorial2: similar, to above: would prefer to use the scipy implementation.
• binom: same

[AlexanderMath]

Could you add a list of the needed functions? Assuming they are things like erfc and inv_gamma etc. Do you suspect we'll have to implement them in C?

[hatemhelal]

I think some of the issues might go away by upgrading to 0.4 JAX (or at least there will probably be new issues). I think there should be some cmath functions that could be used at least for the gamma evaluation within the vertex but would need to be tested.