[ENH] Support multiple thread

[KmolYuan]

Support multiple thread for calling objective function.

• Objective function ObjFunc is read-only and should move to Arc.
• Still keeping IO functions (read & write) as none multi-thread.
• Should be optional for activating thread::spawn. (to be a feature, maybe include in default)

Work on:

• Initialization process of the pool.
• Any parallel like process. (not iterating)

[KmolYuan]

Basic parallel processing is done at v0.8.