Run optimization or verification for different conformers in parallel

[shihchengli]

Currently, the optimization or verification for each conformer can only be run consecutively. It is ok when cheap methods (e.g. semiempirical methods) are used. However, if DFT methods are used, waiting for those steps to be run consecutively is time-consuming. Hence, allowing those steps to be run parallelly for different conformers would accelerate the whole workflow.