taskflow

Merge pull request #338 from fstrugar/master

Fix for "warning C4127: conditional expression is constant", "message : consider using 'if constexpr' statement instead" in VS 2019

parallel_pdqsort's recursive async lambda call to parallel_pdqsort was capturing a reference to Compare on caller function stack which isn't guaranteed to remain valid due to nature of Subflow::silent_async call; made it capture by value instead

updated handbook

updated cuda_error