Interface to analytic continuation toolkits

The ACTest toolkit is not only capable of generating testing datasets, but also of invoking external programs for analytic continuation calculations. Currently, it is integrated with the ACFlow toolkit via the actest/util/acflow.jl script. ACFlow is an open-source toolkit written in Julia. It supports a variety of highly optimized analytic continuation methods, including the maximum entropy method, barycentric rational function approximation, Nevanlinna analytical continuation, stochastic analytic continuation (both the Beach's algorithm and Sanvik's algorithm are supported), stochastic optimization method, and stochastic pole expansion, etc. The ACTest toolkit is designed in a modular fashion, making it not very difficult to interface with other analytic continuation tools.