[spike] More helpful compiler errors when using QtConcurrent

When using QtConcurrent, the user often gets compiler messages like

error: no matching function for call to ‘blockingMappedReduced(...)’

which is not helpful at all.

Maybe some static_asserts would improve the situation dramatically.

As suggested by Marc in the comments below, we need to experiment with the modern compilers to see how the exact formulation of the constraints (enable_if-s now, and C++20 constraints/concepts later) affects the error messages, and find a sweet spot between implementation complexity and readable error messages.