ZMCintegral-v5.1: Support for Multi-function Integrations on GPUs

In this new version of ZMCintegral, we have added the functionality of multi-function integrations, i.e. the ability to integrate more than $10^{3}$ different functions on GPUs. The Python API remains the similar as the previous versions. For integrands less than 5 dimensions, it usually takes less than 10 minutes to finish the evaluation of $10^{3}$ integrations on one Tesla v100 card. The performance scales linearly with the increasing of the GPUs.