Hotfix/pool

[maddyscientist]

Minor hotfix PR

• Expose new interface function flushPoolQuda(QudaMemoryType) to facilitate user control over flushing QUDA's memory pools
• Remove legacy option QudaMultigridParam::setup_minimize_memory which was broken, and redundant in the era of unified memory
• Fix BiCGStab and BiCGStab(l) for use as null-space generators

[bjoo]

This looks nice and clean to me - only the question about the strategy to bomb on OpenMP with the NVHPC++ compiler remains and it is not very important. I am happy to approve this.