Example PR

[TysonRayJones]

Do not accept this PR

This is an example of how to contribute a new funciton to QuEST.

This new function calcAmpSum is relatively simple in several ways:

• the statevec_calcAmpSum implementation and densmatr_calcAmpSum implementations are identical, so only the former is defined
• the distributed CPU version merely invokes the local CPU version and performs additional reduction, so an explicit statevec_calcAmpSumDistributed definition is not needed.
• besides validation, there is no common hardware-agnostic processing needed, so statevec_calcAmpSum is defined directly in the backends, and not in QuEST_common.c

The complications of this function were:

• additional user-input validation (in QuEST_validation) needed to be defined
• in multithreaded mode, this function needed an OMP reduction
• in distributed mode, this function needed an MPI reduction
• in GPU mode, this function needed a parallel & sequential reduction

[TysonRayJones]

This example is make redundant by the architectural overhaul of v4