Change the amout of threads used for a computation

It seems like IVM is using all threads available on a machine per default. When sharing resources on a cluster with multiple users this is not something that you want, because different jobs would have to fight for the same cores.

Is there a way to change this behavior? The amount of threads used does not seem to be related to julia-multithreading or blas.

issue 中未指出文件、测试或入口点。首先在共享机器上复现 IVM 的默认线程使用情况，并追踪它是否受 Julia 多线程和 BLAS 之外的机制控制；完成的标准是有一种文档化的方法可以独立限制计算所使用的线程数。

由索引模型根据 Issue 内容生成。