This is a rewrite (restructuring) of the learner code so that it uses the newly added feature that allows execution of arbitrary tasks on stockfish's thread pool. In particular it:

• removes many parts of the code that were previously required for synchronization and task scheduling.
• provides clearer distinction between single and multithreaded execution contexts
• makes the control flow more linear
• decouples SfenReader from LearnerThink
• removes [no longer] used parts of the learner
• removed old misleading comments and replaced them with new ones where necessary.
• adds Entropy struct to make passing and summing all the entropy statistics around the code easier

As this is a more or less a complete rewrite the diff is kinda hard to read, best to compare whole files side by side. If you think this should be split further I can try, but in my opinion looking at this in its final stage is easier to compare than looking at intermediate steps in isolation.

As a side effect it may have [partially] fixed #164.