`Features`: Support for range-like scalar operations

[clarisma]

Features cannot be used for many of the STL range operations (see #11). Using tests such as std::any_of either requires explicit iteration (more verbose) or creating a std::vector containing the features (memory-allocation overhead).

Instead, these operations could be implemented as scalar query functions directly in Features:

• any(<Predicate>)
• all(<Predicate>)
• none(<Predicate>)
• count(<Predicate>)
• first(<Predicate>)
• one(<Predicate>)

In addition, Features could support forEach (also with the possibility to process features in parallel).

Review

• Naming consistency with toolkits for Java and Python

[clarisma]

(Some of these are already covered by filter() with predicate.)