Harmonized parallelization and functions

[jibarozzo]

Evaluate parallelization frameworks and implement in extract_core() and multi_rarefy()

• future
• parallelly
• future.batchtools

[jibarozzo]

See #39. And @jibarozzo comment: "Looks good. I propose we further refine parallel_* functions. I think the logic can be implemented as new parameters in extract_core and multi_rarefy and we can avoid a whole other set of function. For now, it's good"

[jibarozzo]

@Gian77 let's meet and work together on this.

[Gian77]

@jibarozzo ok, I am not very familiar with parallelly, good idea to meet. Also, will try to run in HPCC.

[Gian77]

Can this be closed too @jibarozzo ?