Split the sigmakin kernel into smaller kernels

[valassi]

This is just a placeholder to discuss the idea of implementing smaller kernels.

A lot of pointers already exist related to this:

• Stefan's WIP PR WIP - Split kernels and more #242
• issue cuda graphs #12 about cuda graphs
• issue Cuda streams #11 about cuda streams
• issue "xxx" function interface: further separation of data access and calculations? #175 about the internal APIs

One of the main points towards using smaller kernels is the need to allow each ixx/oxxx and each ffv function to handle pointers to large buffers for many events and to do the indexing themselves. This is discussed in #175 (comment) for instance. Presently instead only the ixx/oxx functions are able to find an event in the input array, but then their output (and all inputs/outputs of the ffv functions) refer for CUDA to a single event. This is the first thing that must be changed to allow smaller kernels.

[valassi]

Note a basic prototype idea in #313: it would be enough to add for instance bufferAccessWavefunction( fptype* buffer, int iw6 ) wherethe iw6 index is passed in the same way as the ip4 now.

[valassi]

Note that PR #328 is the first step for this and contains several comments related to this

[valassi]

The color algebra optimisation has a separate issue #155. Improving the timing measurements is in #372.

[valassi]

One of the main issues in splitting kernels is the clarification of the relative roles of MEK and CPPProcess: who holds intermediate data buffers? who orchestrates the order of kernels? who is allowed to have process specific stuff? I am discussing this largely in #356, specifically in the econtxt of running alphas issue #373 and draft PR #434