feature: add support for parallelized amplification

[monperrus]

When DSpot is considered slow, one natural solution would be to add support for parallelization, for sake a better performance. Conceptually, the core amplification algorithm contains different natural parallelization points:

• over test classes
• over test methods
• over lineages of amplification
• over Maven multi-modules

[danglotb]

I think one thing to do before parallelizing anything is to investigate: What are the points that consume the most time / CPU execution?

Takes one execution, that we consider too long, but that finish. Implement some points where we measure the time for each of the step (and more) listed in your comment and find the one that consumes the most of the time, and fix it.

From my experience, the most consuming task are the multiple executions of the tests, from far.

WDYT?